Which preamp for Quicksilver mono amps?

I am not looking for any sound improvement. I have not yet thought much about matching KT mono with QS preamp, but since QS preamp is relatively cheap ($1500)

I wouldn’t bother.  Just put the $1500 towards a real future upgrade like the Spatial Raven, Aric Motherload, etc. that are on another level entirely.

+1 on CJ recommendation. 

Sounds like it wasn’t a good idea to consider a QS preamp.

The AI preamp recently had new tube sets from Audible Illusions. The sound got much better with the new tubes, but I wonder how long it will last. Maybe I need to change the tubes every 3~4 years? That was one reason why I was toying around having a QS preamp which I presume last much longer than AI preamp.

It sounds like not that many people match their QS mono amps with QS preamps. I wonder it is because QS preamps are not as good as QS power amps?

There is nothing wrong with the standard QS linestage or full function QS preamp, and particularly so for the price new. People like to try different preamps with different tube sets to yield different results. You might be better off trying different sources and different cables while your existing tubes are settling in more.  While it may not be needed, it's also fun to test in these other areas too fwiw. Best of luck. 

The Cary SLP-98 F1 pre was a fine system when matched to the Mini-Monos. Or for a more budget system I might want to try the QS preamp. Mike Sanders knows good sound.