Preamp possibles for system...

I am currently using a Belles 21A auricap but have been itching to try something else for possible system improvements. The amp is a T-hold S/500 ll with an Arcam CD23 and Polk SDA-2's with RDO-194 tweets. All IC's/PC's are Morrow Level 3/4. My 22'x35' listening environment is slightly harsh with wood floors and a fair amount of glass. Musical taste is quite varied at moderate volumes.

In general, I feel the base or low level frequencies are a bit forward but not crisply defined. Also the high frquencies seem somewhat constrained or maybe rolling off early...using others terminology I have read. Almost as if the singers are behind the band. Not always, but on enough recording's I notice it.

My previous amp was a Nak 7. Everything else staying the same, I recently switched to the T-hold S/500. I literally would have wrote an exact opposit review of the systems traits critiquing it with the Nak. I was certainly surprised...in a VERY good way.

I have read that the T-hold amps liked certain pre's. Here is a list I have gleaned from some research. I have not seen all of these available, so some maybe out of the question. I would be gratefull for any input to try and target my continued refinement.

Sonic Frontiers line 3
Aestetic Calypso
Audio research LS25 mk1 or 2
Modwright 9.0se
Klyne 7