I'm sorry, but there's just no way that your Bryston amp -- or any Bryston amp for that matter -- can't drive this speaker.
This reviewer says he used a 125wpc Music Reference tube amp (I'm guessing the RM-9 Mk2) and he detected no strain whatsoever as loud as he cared to listen. He also states that the spkr mfr recommends 40 to 200wpc, and he suspects any decent 40 watt amp would suffice.
http://www.enjoythemusic.com/superioraudio/equipment/0915/Gershman_Acoustics_Grande_Avant_Garde_Loudspeakers_Review.htm
In my journeys I've heard several pair of Gershman speakers, none of them driven with anything near as stout as your Bryston. And they all sounded great.
IMHO, you are barking up the wrong tree in trying to explain your problem as some sort of Bryston / Gershman power or impedance mismatch. It almost sounds like something is grossly amiss in your setup. Maybe something hooked up wrong, or perhaps a piece of gear in the chain is damaged or malfunctioning.
The other possibility is you have insufficient overall gain in the Bryston / Gershman channels. I say this because you say your other channels are blasting and Gershmans can barely keep up.
Or possibly an impedance mismatch between your PL preamp and Bryston amp. Is the ratio at least 10X? (even higher is better)
I must confess, I'm a little confused about your setup....... Not sure what you mean by "using pre/pro to adjust speaker trim" 
Is this piece of gear in the chain with Bryston / Gershmans? Are you running multiple channels, including Gershmans?
If so, maybe all this extra gear is the problem? I would take out all extra gear -- proceesor, other amps / speakers -- and set things up as a simple 2 channel rig thusly:
source >>>>> PrimaLuna preamp >>>>>>Bryston amp >>>>>>Gershmans
If all gear is working properly I'd be seriously shocked if this didn't kick some serious ass. If the problem returns when you add in the extra channels, then there's your problem.
Have you tried Gershmans with other amps?
Have you tried Bryston with other speakers?
These steps will help you to logically deduce where the problem lies.
Appreciate you taking the time to reply. I wrote a lengthy response but it seems to have been lost in cyberspace. I'll try to recreate it here...
Yes, in an attempt to procure a quick response, I glossed over my system. It's really two systems in one. One for two channel listening and one for HT.
System 1:
Oppo BDP 103
Auralic G2 Streamer
Denafrips Ares II Dac
Primaluna Evo 400 Preamp (Tube)
Bryston 14B3
Speakers: Gershman Grand Avant Garde, B&W 801 S3 and a pair of Steve Nugent (Empirical Audio) modified Kef 104/2.
System 2:
Outlaw 976 Pre/Pro
Simaudio Aurora five channel HT amplifier (5 X 250W)
Various Bookshelf Speakers as surrounds
Both systems connected to a Bryston BIT20.
When listening to two channel, the only components running are typically the Auralic G2 into the Denafrips and then the Primaluna Preamp to the Bryston. When the wife comes down and wants to watch a movie, I turn on the Outlaw Pre/Pro and set the Primaluna Preamp to HT mode. In this condition, the Outlaw Pre/Pro controls the volume of Primaluna Pre which again, controls the front two channels.
When I was babbling on about trim, I was referring to the initial set up of the Outlaw. A properly tuned surround system needs all speakers to be calibrated via a test tone. On any other speaker, the front two speakers, being run via the Primaluna and the Bryston can easily match the surrounds, being powered by the Simaudio HT Amp. Unfortunately, when the Gershmans are put into the system, there is not enough adjustment available. Ie. I set the "trim" on the front two channels (Bryston/Gershman) to the maximum setting and the remaining 5 channels (Simaudio/Bookshelf's) to the minimum setting, a noticeable split still exists between fronts and surrounds.
I'm afraid I'm out of my league on this problem. I've tried everything I can think of and still, the Gershman's just suck power in and do not give it back. All I can say is that when any other speaker is connected, they sing. When I connect the Gershman's back up, they die. I've tried Balanced, Single Ended, 29db gain, 23db gain, three different preamps and two different amps. I've bi-amped them, I switched polarity (As per Gershman) and still they continue to be the hardest speaker I've ever seen to drive.
I have to think that their is some kind of problem with the speakers themselves. I've spoken to Gershman at length and while they believe the speakers are functioning properly, they have graciously agreed to take them back and give them a complete physical.
I have a friend with a JR Model 8TiHC I'm hoping to try and perhaps I can find someone local that may have a big Krell or similar but saving these two last ditch options, I'll just have to wait until I get a chance to head back to Toronto. In the mean time, they still make the most beautiful music when the volume is at 50% and I'll just have to get used to seeing the volume dial higher than normal.
Again, can't thank you enough for taking time to respond.
Cheers!