I actually tried ~that.That is why tube buffers were invented.
Both the tubeamp and the 120 SINAD amp have pre functionality. So I tried both ways:
- 120amp_as_pre -> tubeamp
- tubeamp_as_pre -> 120amp.
For some reason I preferred the sound of the first combo. It's also more comfortable and the volume-knob of the 120amp is probably better (more linear etc..). Did not do any measurements or serious tests, I just liked it so.