Since you're considering a JVM, have a look at the workbench forum over at JVMforum.com, here:
http://jvmforum.com/phpBB3/viewforum.php?f=7&sid=4b23654bb6a28c441ad247b702a72dd0
In particular click on the Modding Bible towards the top. In that sub-forum you'll find the Mod Catalog, here's the direct link:
http://jvmforum.com/phpBB3/viewtopic.php?f=31&t=5252&sid=d99616c7cb7de730be2a2ad1d2033869 it lists the most popular mods on the forum and gives a basic rundown of how each is done and what it will do to the amp.
I've done the Blackface Clean Channel, 1959 Plexi Crunch ChannClean Plexi One Wire/3rd Gain Stage, JCM800 2203 OD1, Rectifier OD2, Negative Feedback Adjustment, and Plexi Cap mods to my 410H. They all improved the amp, although the Rectifier Mod doesn't really make it sound like a Recto (I own one), but I like what it did to that channel. I also may do the Choke Mod, mod the Poweramp to take 6550s, and might try different transformers in the future. But for now, I'm happy with it.
I did all these mods myself, but if you aren't comfortable, there are guys on there that do perform mods, or any competent tech should he able to do them for you. And all these mods can be done to a 205H, you'll just be a little more limited on how many you can do (you can do any of the mods, but you can only do so much per channel). I guess if I had a 205, I would do the Plexi Cap, Plexi One Wire/3rd Gain Stage, Negative Feedback, which effect the whole amp, then the Blackface Clean to the 1st channel and the JCM 800 2203 to the second channel. The One Wire will allow you to dial in some extra gain in the preamp (quite a bit more, actually) and the Negative Feedback will allow you to get the Poweramp to break up at lower volumes.