The system as a whole is a little (not much) "laid back" for lack of a better term. To me, that’s a lot better than bright, but I’d like a little more clarity and liveliness.
You’re most likely already aware of this, but just to be sure I’ll mention that the symptoms you are describing can occur with a passive preamp if the length of the cables connecting it to the power amp is not short, and/or if the particular cables do not have low capacitance per unit length.
Adverse effects can also occur with a passive preamp if the input impedance of the power amp is too low, but it appears in this case that the input impedance of your amp is 100K, which at least in theory should be fine. But nevertheless I wouldn’t be surprised if Scott’s (Smrex13’s) suggestion of an active preamp would be more likely to be beneficial than an amplifier upgrade, assuming that the amp doesn’t have any age-related or other condition-related issues.
Also, do you sense that the symptoms are similar when you use the Teac as a transport compared to when you are using the Dell as your source? And how are you connecting each of those sources to the DAC (e.g., optical or coax or USB)?
Regards,
-- Al