Was that the same DAC unit? I know tubes can make a difference. Another more likely issue might have been the input impedance of the pre-amp or head-amp. The SFD-1 I have barfed into an headamp which I think had a 10k pot or something like that. The SFD outputs are cap coupled and the caps may have been too small for effective transference of bass signals.
O2: Building and listening to this amp as a noob (a rabid minion, I was) is what diverted me to path of "subjectivism". A combination of inner-harshness + op-amp veil + compressed dynamics