Thanks for the details and link, I'll grab a few on my next mouser order.
SIP8 pin thing makes sense, cheers for the explanation.
Those OSCON caps look interesting, are they more durable than normal electrolytes? Do you think they sound subjectively different? I think I need to look them up. In hindsight maybe it would've been wise to replace at least the 0.47uF caps with film. I also looked at the 4.7uF ones but getting them in film would've been kind of expensive and they tend to get large. But 'expensive' is kind of relative and with the amount of money we are talking, I do think the biggest expense is the time invested in changing them, so...
Yeah they have a longer lifespan (5000h) and higher temperature rating (105*C), but the other reason I used them is that they have significantly lower ESR than liquid electrolytics. I think subjectively in terms of SQ they sound slightly cleaner but the difference is small.
I used Wima MKS2 wherever possible (0.47, 2.2, 4.7, 10uF from memory) and Panasonic ECQ-P for the 2.2uF 100V (C227-C230).
I think using Nichicon electrolytics everywhere is absolutely fine though, and just a general recap (pre-OSCON) did make things sound cleaner across the board just like you said. In my case the amp is almost always in Class-A and sees heavy daily use so I just wanted it as a durable as possible.
In my opinion (re: temperature ratings), it's far better to give the amp good ventilation / leave it open in Class-A mode than just using 105*C caps. I feel 85*C caps are fine, and that good ventilation is more important (for solder joints and other components).
One thing I'd like to try in the future (something Oilmaster suggested may offer a minor audible benefit) is replacing the fluoro green caps often across fusible resistors (and maybe the resistors themselves as he also suggested, they do seem to drift in spec a little). I believe the stock caps are polyester/mylar, whereas ECQ-P are polypropylene. I'm sure the improvements are marginal/nonexistent at this point but it's part of the fun to try.
I don't have a cap meter, but getting a cheap one would have it's uses at times and it would be interesting to measure all the original caps just out of curiosity.
You could get a cheap one off ebay, it's called the Mega328 (~$12). Something I found interesting (and annoying) is that virtually all new Nichicon caps measured below their labeled value, eg ~920uF for a new 1000uF KZ cap. I know that's within spec, but on a few occasions even the ESR was higher than the original 30y/o part (I found this on those Nichicon ES caps for the EQ board). I'm sure it's no big deal, I used them anyway.
For whatever it's worth, I found on Yamaha amps with (large) factory-fitted Blackgates, they still had capacitance higher than spec, and ESR lower than any replacements I could buy. Perhaps the BG cult following was justified!