I used to own the 20 amp PM2000 prior to moving to VTL tubes. The unit I had was not the latest version, but there were never any problems with humming or excess transformers noise which the Plitron transformer eliviated.
I was told the Plitron transformer should labled on the transformer itself. Most of the later V-cut pm2000 had it and some of the straight cut units had it. Some of the other changes on the newer version also had better speaker binding post that looked like WBT, but I don't think were.
You may want to asked the seller to check under the hood or call Alpha digital in Oregon and see if they would know.
If you go through with the purchase the PM2000 and it does not have the Plitron transformer you can be upgarded to have the Plitron transformer and even converted to the 20Amp version which is a BIG plus if you have a dedicated 20amp circuit as I did.