Your best and cheapest bet would be a Slot 1/Socket 370 ATX board since you can use modern PSUs and cases with those, which simplifies things a lot. There are mATX ones as well but more rare and expensive. If you get a ~500MHz Pentium III Coppermine for it passive cooling will work out great.

taking in account you want to underclock, a slot1 mboard with a 666MHz PIII would be a good idea, just down the fsb from 133 to 66.

As I understand the requirements for Adlib Trackere a very slow Pentium should be sufficient. (I don't know about your other music programs.) So maybe you could have a look at (super) socket 7? At least those boards should all include an ISA slot.

Ideas what CPU to use:
The K6-III+ 400ATZ is AFAIK targeted for embedded environments without active cooling (eg. cash registers). I think this type has been discussed before in Marvin, it shouldn't be expensive on Ebay. You'd need a board that supports 1.6V core voltage, though. I think 100MHz FSB support wouldn't be important for you but boards that support the needed voltage would have that anyway. You could underclock the CPU even further.

IIRC I accidentally underclocked a Pentium MMX 200 using settings from a K6-2. The settings would have been 66MHz FSB, x2 multiplier (making the CPU run at 133 MHz) and 2.2V core voltage. I don't remember if it ran stable but it booted. (Since the MMC 233 still uses 2.8V core voltage this doesn't seem too strange to me.)

SS7 stuff is quite a bit more expensive and hard to find on ebay.

I would recommend a BX board with Celeron 100 MHz bus support. Then you can underclock it to 66. I have such a Celeron, at 800/100, but it runs quite hot without a ventilator. Never tried to underclock it at 566 though. Should run cooler.

For Adlib Tracker any Pentium 1 or better will do, though once you start using many effects and macros at a high tick frequency, even a P3 will have trouble keeping up.

And in my experience a slot Pentium 2/3 with sufficiently large heatsink doesn't need any fan at all, at stock speed.

d1stortion wrote:

SS7 stuff is quite a bit more expensive and hard to find on ebay.

Depending on where you live that's probably true. Of course it depends on what parts you might already have. CPU and RAM should be easy to find.

So if you'd take a PIII with FSB 133 you could underclock it to FSB 66. The fastest speed you could get this way would be with a PIII-S 1.4GHz running at 700MHz. It just might be a bit more difficult to find a motherboard which supports Tualatin that older PIII's.

Still, the most important part about underclocking is to enable you to undervolt as well. So of course a motherboard with undervolting support would be desirable (my GA-6VTXE just has solder pads where the jumpers/switches should go).