If you have an issue with your speakers audibly driving the floor and room structure then isolating the speakers will bring improvements. Many people don’t have an issue. Most expensive audiophile “isolators” don’t provide effective isolation but, surprisingly for expensive audiophile accessories, one or two do. It would likely be cheaper though to use products sold to address acoustic installation issues with machinery.

Off except when someone forgets. Not religious about it but why would someone want their hardware on all the time?

It depends to a fair extent on what you want it for. For a fuss-free first main hi-fi system in the lounge it is hard to recommend unless it is free or close to free. To reinforce what has been said by others, it is a budget design from the early 70s with a bit too low power for current hi-fi speakers, specs that are modest to the extent of possibly not being audibly neutral in use in some situations, components may be failing or are about to start failing due to the 50 year age (all of my amplifiers from the 70s developed problems and were passed on or scrapped). On the other hand if you want to mess about learning to maintain old hardware as part of a hobby interest then it may be worthwhile depending on condition.

All but two of the hi-fi shops I have been to in the last few decades have been helpful with a couple being extremely helpful. In the first exception I was refused a demo of what I had gone to buy. It could be seen as arrogance but at the time I was more baffled than annoyed because it seemed so weirdly counter productive. I guess I must have been judged a tyre kicker for some reason but it was odd. In the other I suspect the salesman genuinely held a range of the more extreme audiophile beliefs which made communicating with those a bit more grounded problematic. This could come across as arrogance because the lack of rational arguments to support those beliefs tends to lead to defensive behaviour with bald statements of “fact” and an unwillingness to consider conflicting information.

I have never been a salesman but if I “knew” the person in front of me was a time waster rather than a genuine customer then acting in an arrogant manner might be an effective way to make them go away and stay away. I never returned to either of the shops where I received an unhelpful response.

Depending on thickness they will lower the level of some of the high frequencies but not the mid and lower frequencies degrading the spectral balance of the reflections. What won’t change is the lack of symmetry generating different arrival times for the early reflections from left and right which is likely the main negative contribution. It should make differences but whether the sum is positive, negative or about the same is difficult to say.

Moving the balance control away from centre should improve the centring of the imaging. Individually EQing the channels should help improve the different boundary loading (and the room response potentially). The differing wall reflections will adversely affect imaging, tonal balance to some extent and spaciousness to some extent but cannot be effectively addressed electronically and will have to be lived with.

A conventional driver has a linear frequency passband of about a decade limited by being small enough for resonances at the high frequency not to intrude and large enough for the low frequency end to be loud and clean. This leads to a 3 way and, indeed, pretty much all professional midfield monitors are 3 ways.

A 2 way requires too wide a passband from a conventional driver with the result the low frequency end tends to be limited both in extension and clean SPL plus audible midrange deficiencies due to midwoofer resonances, tweeter distortion at high SPL and often directivity issues. Nonetheless a 2 way is normally significantly smaller and cheaper and can be made to work reasonably well just not clean at sufficiently loud levels over the full frequency range. They tend to be the better choice for budget speakers but once the price level is above about £1k (perhaps a bit more these days) a well designed 3 way is going to provide noticeably better performance than a well designed 2 way with a bit more expensive components.

The minidsp and avr both have the necessary high/low pass filters on the issue is they are still audible at the port even though they shouldn’t be.

This would appear to be what might need getting on top of first. In normal circumstances the low frequency sound quality of speakers is all but irrelevant in the presence of booming room resonances. If these are addressed with distributed subs, careful signal processing and some passive room treatment then one might just about be able to discern a tiny effect due to group delay in ported speakers with a fairly high tuning frequency. With a normal port tuning of 10-12" subs the lower distortion and higher SPL will dominate perceived sound quality which is why effectively all professionals subs are ported with only a tiny number sealed.

BTW not knocking sealed subs given my subs are sealed. I would have preferred the performance advantages of ported but they would have been too large to distribute enough of them to adequately control the room response in my room which is both asymmetric and not large.

If you were to integrate your subs “properly” with a high pass filter on the mains matching the low pass filter on the subs (i.e. so that the mains and subs sum to a flat response rather than lumpy and the IM distortion in the mains is reduced) then the loading of the woofer wouldn’t matter.

It seems to me that your source would be the most important given it’s the origin point.

Why?

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Of course, any system is only as good as its weakest link,

Indeed but how would you square this with the first statement?

The source of imaging is in the recording but we can assess how the room and speaker geometry will change/influence the perception of image location and to a fair extent, though not completely, the perception of a sense of image width and envelopment. It involves simulating the propagation of sound in the room (a lot of work and some expense but not a problem these days) and quantifying the relevant psychoacoustics (trickier but doable in a less precise way).

Academic journals are likely to be the main source of information with possibly the odd text book. As you can see from the responses here there is not much interest on audiophile forums like this in what is going on in a scientific sense when it comes to sound quality. You might be better off asking in a forum for acoustic consultants. There used to be a reasonably active one on usenet but that petered out many years ago. The acoustics forum here contains one or two posters with knowledge of acoustics but their posts tend to get swamped by room acoustics noise and enthusiasm much of which is unreliable. Might be worth a post there if you have an interest in pointers towards the science side of things.