what impacts the sound quality more amp or pre-amp

"The Alon II speakers are specified as 87 db efficient and are, I think, 4 ohm nominal impedance. This is a difficult load for most tube amplifiers to drive."
No mention is made of phase. This would be the deal breaker for tubes, that and wide swings in impedance.

4ohm and 87db are nice, but not deal breakers for tubes. Harbeth, less sensitive and considered tube friendly.

Tube amp choice, however, DOES imply certain speaker choice.

pre is the heart of the system. As said balancing it all out helps more.

I think the choice of speaker and then the choosing the amp
to drive them is the most important choices in building a
system; other than the recording and the room, they will
have the biggest impact on the overall sound of your system
- the change brought by digital source and preamp, while
important do not make nearly as much of a difference as the
amp/speaker in my experience. I also agree with Wilysnet
recommendation on an excellent tube preamp selling for a
fraction of the cost of its acoustic competition, though if
I had a tube amp I would go with the even less expensive
Lightspeed Attenuator $450 which is the best sounding volume
control tool I have ever heard if well matched to an amp
(though not very flexible with only one input/output, and no
remote control.

So yes, everything is important....but the amp/speaker is
critical and the most defining pairing for what you system
sounds like. Once you get that right, the rest is relatively
easy (though not foolproof).

would a moscode 300 hybride tube amp pair well with my system?

It would drive your speakers, but seems like a lateral move from the B&K, I would also worry about such a old amp unless it gets the once over by a tech and possibly its capacitors replaced - it is a early 80s amp - no?

Could you describe what it is that you are trying to change with the sound of your system? Something you don't like about what you currently have that you are trying to improve? That B&K amp is a pretty nice SS amp.