Hi D
If you don't want to mix and match, Bryston's own BP20 preamp is a safe match for the 4B, if maybe a bit solid-state sounding. If you choose a tube preamp, you generally get a more liquid sound, which goes well with the 4B. You may get some roll-off in the lowest bass, depending on which tube preamp you get. With the Kef Q65 and the 4B this may not be a significant factor.(I have not heard this speaker before, just read a review) I use a Golden Tube SEP-3 tube preamp with a pair of Bryston 7BST's and find them a great match in terms of musicality and resolution. I did have to play around with cabling, though, FWIW.
Good Luck
James50
If you don't want to mix and match, Bryston's own BP20 preamp is a safe match for the 4B, if maybe a bit solid-state sounding. If you choose a tube preamp, you generally get a more liquid sound, which goes well with the 4B. You may get some roll-off in the lowest bass, depending on which tube preamp you get. With the Kef Q65 and the 4B this may not be a significant factor.(I have not heard this speaker before, just read a review) I use a Golden Tube SEP-3 tube preamp with a pair of Bryston 7BST's and find them a great match in terms of musicality and resolution. I did have to play around with cabling, though, FWIW.
Good Luck
James50