Damn without budget restrictions I would say ENGL Savage 120 or Splawn Nitro or Quick Rod. But plenty of good suggestions and I'd say the 5150/6505 route is solid. Thrash Metal bands seem less on the 6L6 tone though, prefering cutting mids and crunch over the low end rumble. I'd retube whatever you get with KT88s or 6550 power tubes; a lot of thrash bands run amps on those tubes. EL34s are a good option as well; I just prefer the KT88s or 6550s. I had 6550s in a Peavy 5150 sig combo, and KT88s in a Meteoro MAK 3000 (facking 280 watts of MASSIVE dark tone; open chord palm mutes sounded like an anti-aircraft gun, and felt like the overpressure!).
My case for those tubes has credit: MAK 3000 is the Andreas Kisser amp, and Exodus recorded rhythym tracks with the Savage 120. Plus back when he was in business Derek of Rock Solid Amps of South Florida had a tone of awesome sample tracks of their boutique heavy amps and they nailed several different thrash tones, especially 80s thrash like ...And Justice for All and Master of Puppets, with the ENGL Savage 120, Fireball 60, Thunder, and Screamer. It's a damn shame they went out of business (and took down the website). There were plenty Exodus, Megadeth, Overkill, etc clips on that site each nailing the said tones with their own character added.
