Jesus.. I have no desire to even try an EQ in the loop, this thing has an ASSLOAD of low end! I put some Ruby 6L6GC-MSTR's in it, rolled some preamp tubes, (a Tungsol in V1, JJ ECC83S's in V2 & V3, and a Sovtek LPS in the PI). This thing is near perfect with the bass and depth at noon, presence up a little higher, treble down a little lower. The Tungsol brightens it up, and is much tighter than the stock EH 7025's. The JJ's are tighter as well, yet warm, less round, (less woofy) than the EH's.
As per my suspicions, the SD-1 wins out as the boost of choice. I also run a Boss CS-3 Compressor after the SD-1, and it's so damn good sounding. And as many have said in YouTube videos, and reviews, it only gets tighter and punchier when you get the volume going. This is a fabulous amp. I absolutely love everything about it. Tone to die for, and it hits like a sledgehammer!