I used to have four PM-180's. Yes, there's a Tamura transformer for each mic input and the main outs, but these are all different than the ones you'll find in other, bigger Yamaha consoles of the same vintage.
I've never done an A/B between the PM-180 types and, say, the PM1000 or PM2000 types, but I can say that they are tiny compared to the ones in the bigger boards. Same 1:3 ratio, however as the larger boards.
The mic pre's use a Toshiba 7-pin SIP IC package that is capable of 24-volts, though the PM-180 (and 170) use bi-polar 22.5 volts. The sound interesting, and actually compared pretty nicely to a class-A Neve in size, but the sound was a bit more smeared somehow... didn't maintain its authority when placed in the mix. My guess is either cheap transformers or cheap op amps in the channels.
Also, the eq on the thing is interesting as it is a 2-band (hi/lo) eq with fixed frequencies, but they're not shelving filters, they're peaking. Also, the HF band uses a real inductor and actually sounds pretty cool. I imagine with some creativity and elbow grease you could make an interesting and useful device out of it. I thought about it, but decided I just didn't have the time and sold them off.
The best thing about the PM-180/170 is the two discrete output amp blocks, parts NE80100, I believe. These are the line-level version of their discrete op amp of the time. Both this amp block as well as the mic version (NE80200) are used in the PM2000, and many people have spoken about how good that board sounds. Different (bigger) transformers in the PM2000, however.
It may not be a 2520, but you get two for your $50 purchase price of the PM-180, and that's a lot cheaper than a pair of 2520's (if you can find them) for $75 each and up....
Also, the Yamaha amp blocks can run on +/- 24 volts, while the 2520 tops out at +/- 20 volts.
The Yamaha amp blocks share the same pinout as the 2520 (and just about every other discrete op amp from Hardy, Jensen, Melcor, Inward Connections, Forssell, Millennia, etc...) I still have a couple of these amp blocks an intend to try them in a couple of channels of Fred Forssell's opto compressor when I can find the time.
Peace,
JC