good preamp for First Watt F7

See bellow the answer of Mr. Pass:
The F7 has a lower input impedance than most, at 10K, and a little less gain than most. Preamps that would have difficulty would be ones where the output impedance is greater than 1000 ohms or so, or tube preamps that cannot driver 10 Kohm, or which have no voltage gain. As it is, I routinely use the the F7 with a B1 which has no voltage gain, but my sources have 2V or more.

I am starting questioning do I really need a dedicated preamp to improve the sound. My PS Audio DS has  gain of 1.414 VRMS for Single Ended outputs (F7 has SE only inputs) and  Output Impedance 100Ω/200Ω . 
Looks like  reasonable preamp for F7! 
I use DS as my only source ( DS CD transport connected to it), so I started looking for preamp because I've been told that both F7 and DS will benefit from preamp . 
I guess I'm opening the can of warm "Do you and when you need a preamp" 

As it is, I routinely use the the F7 with a B1 which has no voltage gain, but my sources have 2V or more.

A lower output impedance than the B1’s 1kohm "may" sound better into the F7’s 10kohm
Maybe trial (14days) the $299 active unity gain Saga-S or one of the others, you can have active tube/solid state or passive and remote. Send it back if not happy
https://www.schiit.com/products/saga-s
Cheers George

I used a Manley shrimp (well matched impedance for a tube pre),  wyred4sound STP SE, and B1 (DIY) preamps with my F7, first 2 worked great, better than the B1 to my ears. Good luck!