Listening more closely to my set-up, I think the distortion I mentioned early is starting with the Sorado not the speaker in the SVT-15E. This is with the volume on anywhere above 4. I've been playing with the EQ with the bass between 4 and 6 and the treble between 2 and 3 (I'm leaving both hi and lo boost switches off). Above 4 on the volume, the low end starts to compress with some distortion and the midrange, especially the upper mids, start to come to the fore.
I really like how the amp sounds on volume 3 and the bass on 5/6, treble on 3. It's warm and clear with a slight chime on the top end with a really great dimension and enough bottom. Once I start getting above that on the volume, I'm not sure how I feel about the compression I'm getting. It sounds nice in an abstract way, and I think would be useful in band context with a drummer, but that's not my sitch. I was expecting the Sunn to stay cleaner with the volume up. Of course, the little guy is only 60-70 watts so I might be expecting too much from it in that regard.
Seems like I either need:
1) a different tube amp with a bigger power section and a voicing that's designed to stay relatively clean.
2) more cab. I've got it on a 1x15 with pretty good sensitivity, but with such a small power section, seems like there's only so much I can expect.
3) to coax a bit more headroom out of it with a different preamp tube? It's got one 6AN8A in it. I'm new to tube amps and I don't have any experience with preamp tubes outside the 12AX7/AU7 family. I'm not sure how much difference this would make.
Any thoughts from those more familiar with tube amps in general or the little Sunns in particular are welcome. In general, I'm just going to keep experimenting to see if I can find that full-range tone I need from this set-up.