I believe the GBE 600 is discontinued. However I thought it was just FET with no tube channel. I think you had to go up to the GBE 750 to get a tube channel?
-Is the Shuttle 6.0 "Warm/Full" as the OP required ? My limited experience with the Neo Pak 3.5 (same voicing) was pretty high mid, bright. But the tube did add grit.
-I was thinking a Walkabout head (used) meets your requirements. Always thick and warm but can get clean-to-super gritty by adjusting the Gain. It's Powerful EQ will get you bright and twangy but the warm, thick nature of the head keeps the bright,twangy from being too offensive.
-Or for more of an SVT like grit you may want to watch for a Mesa Big Block 750 (used).
Yes, the GBE 600 is discontinued, but you're mistaken about it's preamps - both the GBE 600 and GBE 750 have both FET and Tube preamps - the main differences are that the GBE 750 has separate EQ sections for both preamps, and is 750 watts @ 2 ohms, as opposed to 625 watts @ 2 ohms for the GBE 600...
AFAIK, the above info is accurate - but I'm no expert, and wouldn't want to mislead anyone in any way... however, I have been playing a GBE 600 for a little while, and know a bit about them...
- georgestrings