Preamp Suggestions for Monarchy SE-160 amps???

If your current preamp is only flat sounding, I would keep it for now. Its very hard to find a good, inexpensive preamp in your price range; new or used. I would consider a passive solution but not active. If you go active, you are likely to get something else that sounds flat; or worse, you'll end up with something that is bright or harsh. The preamp is too just too important. The only cheap way around it is to not use one.

The only cheap way around it is to not use one.

Not true at all.
Check out Mapletree Audio http://hollowstate.netfirms.com/
2A SE Linestage: Point-to-point wiring; customizable gain for each input; separate PS; 6SN7/12SN7 tubes switchable; single volume and separate channel level controls; best customer service; new < $1k.
My system retails for over $15k. Some say the preamp is the most important system component. My 2A SE Linestage, as inexpensive as it is, is not going anywhere. I love this piece - it is wonderful.

The B&K Pro 10 works real good with your monoblocks, but its hard to find one.