That being said, are there other factors aside from harmonic and intermodular distortion that might account for different house sounds? Are there other factors which make different amps sound different, even with negligible distortion of any kind?If the amp is not using enough feedback then bandwidth and related phase shift will also play a role. For example a rolloff at 50KHz can make the amp or preamp sound a bit dark. This is because the phase shift can go down to 1/10th the cutoff frequency. In the same way, a cutoff above 2Hz will affect how the circuit presents bass. If the bass has phase shift it manifests as less impact, up to 10x the cutoff frequency.
This is why wide bandwidth has been such a big deal. But if you have enough feedback, then you don't need so much bandwidth because the circuit can correct phase shift as well as distortion.
What a genius and I know that he voiced the Silver 7s 900 on Apogee Scintillas and the 350s were voiced on that ( I have a pair of Crimson, the only color choice they should have IMHO)) and now voiced the 275s on those. Great clean bass and extended highs. I believe but can't prove he voiced his amps based on his ear, not on so-called industry or acoustic specs, etc.Its unlikely that the amp was 'voiced'as you describe. As an amp manufacturer I hear about this sort of thing a lot. But 'voicing' really isn't a thing; if you compensate an amp to work a certain way with a certain speaker, you've created a synergy. Another way of putting this is you have one distortion put in place to offset another distortion (and probably one of an entirely different type), which results in compromised performance. So I suspect what is true is that Carver simply used the Apogees to listen to the results of his work. You do the math to design the circuit, but you'd be an idiot if you didn't also put the finished circuit on the bench and see if it needs any tweaking, since the math and the actual parts often aren't the same thing. After it performs well on the bench then you play it and see what you think. But this process isn't voicing because you aren't making changes to make it work on that speaker. You're making changes so that the measurements show you its worth a listen.
Now there is another issue, that of the spec sheet as opposed to what we hear. If you are pragmatic as an engineer, you know that the rules of human hearing are not entirely taken into account on the spec sheets. This is why some amps can measure poorly but seem to sound just fine. Robert Harley commented about this in an article (about amplifier types) for Stereophile a few years back but made the comment (about SETs) that 'no-one knows why' they can sound so good and measure so bad. If you've been reading what I wrote here and in my post above, you can clearly see he only made that statement out of an incorrect assumption that designers didn't know why that can be so. Its certainly possible and advisable if you want to make a listenable amplifier to keep the human hearing rules in mind and design for them rather than spec sheets that ignore how we hear.
(One way spec sheets ignore human hearing is showing a low THD number which looks great on paper. But if that low THD is entirely higher ordered harmonic content the amp is going to sound pretty bad - it will be bright and harsh. If you want the amp to sound right, you have to either keep the higher orders way way down or have enough 2nd and 3rd harmonic to mask their presence. You also **at the same time** must have the same distortion percentage at 100Hz, 1000Hz and 10,000Hz. Many solid state amps have excellent distortion figures at 100Hz, which is why they are usually measured at that frequency. But if you measure at higher frequencies, like 10KHz, you find that the distortion is higher. Amps like this are brighter and harsher on this account, even though they have 'low THD' on paper. This phenomena is caused by insufficient Gain Bandwidth Product to support enough feedback, so as frequency goes up the feedback amount goes down.)
Now if you're going to say 'that's voicing' when the idea is perhaps to design to keep the higher ordered harmonics inaudible then I'm going to agree, although I simply see it as measuring properly. But that still isn't done for a particular loudspeaker.