I also can get quite loud volume without the preamp.If it's loud enough you don't need more gain.
I talked to Roy Mottram at tubes4hifi. We are going to try a buffered passive preamp.No need, you have 50kohm input impedance on your amp, a $49 Schiit Sys will do the job perfectly.
And without any distortions/colorations the buffered one will introduce (and it’s probably just an opamamp buffer)
Cheers George