Why is my amp requiring as much volume gain?

Just swapped out a pair of 1980 vintage Denon 1500 (150 watts stereo I beieve)in horizontal bi-amp configuration (using little XO crossover box).Know that amps have no mono conversion so it must have been horizontal (or no crossover would be neede as well).Two stereo amps combined in that fashion should be putting out 225 watts total correct?But then put in Bryston 4BST which is rated 250 and benches closer to 300 wats.But need to turn up my pre to 12 o'clock high to get what I'd get at 9 o'clock.Only thing I can think of is that the input sensitivity of the Bryston is much lower than the Denon's.But that much?Haven't tried to see if I really punch it but the Bryston SHOULD be louder or at least not require so much more of the dial.Guess if I do really turn them up (which I haven't yet)my guess is that in addition to sounding better harmonically (and no cut outs/crackling of course.Think Bryston is working as it should.They are as everybody say's prety indestructible)I expext it to be louder.But is the lower input sensitivity it?Why off so much versus less power and can I adjust it if I feel I am running out of dial room on my Mac 172 pre?
I think you've answered your own question re: input sensitivity. It is only really a problem if at max volume on the preamp you still don't have enough gain to satisfy your in-room max dB requirements.

One thing to keep in mind is that most preamp volume controls are more linear in the middle and upper control ranges and could possibly sound even better at the 12 o'clock position as opposed to the 9 o'clock position.
The difference in wattage output translates to less than 3dB in added volume potential. That's only a little bit louder at full volume. It's just not a dramatic difference.
The Bryston amp has an input sensitivity switch that selects 1v or 2v input voltage. Check with the owners' manual or Bryston to see if you're using the correct setting.