Have I Hit The Point Of Diminishing Returns?

Looking at your system description leads me to ask if you are connecting the Velodyne sub at speaker-level or at line-level. If you are connecting it at line-level you could undoubtedly realize a significant improvement, at minimal cost, by connecting at speaker level.

I say that because, as I mentioned in another recent thread in which you also participated, the output impedance of your BAT preamp rises to very high levels at deep bass frequencies. The input impedance of the line-level inputs of your Velodyne sub doesn't appear to be specified, but typically powered subs have relatively low impedances on those inputs, which would result in significant deep bass rolloff when driven by your preamp. Reproduction of higher frequencies by the main speakers could be adversely affected as well, if the sub is loading the outputs of the preamp.

Regards,
-- Al

A thought to consider: Rather than fixing what doesn't seem to be broken (i.e., "upgrading"), consider adding a high quality pair of headphones and a headphone amplifier. I find having both speakers and headphones available (Stax electrostatics, in my case) to be nicely complementary. For several reasons:

1)Having two (very different) sonic perspectives adds interest from a musical standpoint.

2)Headphone listening takes the room, the speakers, the power amp, and most of the preamp circuitry out of the picture (assuming the headphone amp is connected to the preamp's tape outputs), which can be helpful in diagnosing system issues and/or providing reassurance when audiophilia nervosa strikes.

3)Headphones, of course, can make it possible to listen when listening via speakers would disturb others in the household.

Just a thought. Regards,
-- Al