Where for a tube-newb to start?

That's a nice little amplifier - mostly point-to-point wiring, about 25 watts per channel. You should be able to do some tube rolling with it. There are a variety of EL34 tubes, and you might be able to use KT77s as well, which could give you a little more extension top and bottom. There are reviews on Dagogo, Possitive Feedback and Six Moons. I agree with Brownsfan, despite his questionable taste in football teams, don't discount the importance of the rectifier tubes. They definitely can have a positive effect on the sound.

@Whart: Thanks for all the information! I have received the unit and found that it was populated with that appear to be stock tubes. Frankly, they sound awesome with my Spendor SP1/2e speakers (also inherited with the M520). I’ll be quite happy listening to them while I find “rolling” alternatives.

@Tbg: The amp has Eastern Electronic “Performance Grade” tubes (5 per channel: EF86; 12AU7; EL34 [x2]; 5AR4). I’ll be checking audioasylum as you suggest.

@Zd542: Thanks for the link to The Cable Co. They do appear to have some reasonably priced replacements (e.g. Mullard EL34’s for $37.12 each).

@Brownsfan: Thanks for the recommendations.

@Grimace: I’ll definitely have a look at the resources you site.

Cheers… Brian

"@Zd542: Thanks for the link to The Cable Co. They do appear to have some reasonably priced replacements (e.g. Mullard EL34’s for $37.12 each)."

Glad that I could help. I've done business with Cable Co for many years and find them to be extremely trustworthy.