Bring my own preamp to an amp demo?

I'm going tomorrow to demo a Bryston 4B3. I have a PrimaLuna Evo 300 preamp and Harbeth 30.2 XD speakers. The dealer suggested that I use a similarly priced preamp that he sells, but if I want to hear the amp with my specific preamp and the speakers I own (which he sells, and I bought from him) I need to drop it off when he opens and come back later because he has other appointments booked and needs time to connect my PL preamp, and maybe (I'm guessing) get it warmed up. I'm also going to listen to a BAT VK-56SE, and he suggested I keep an open mind and listen to an Einstein Integrated amp and thinking about selling my PL Evo 300 preamp. He wants me to have an open mind, and said he has a lot of customers that like and bought the Einstein with Harbeths. He also has an Audio Note amp demo for me to listen to as well. He's not a fan of PL, but is happy to work with me.
Bring my preamp or not?
I've had dealers let me give them my card as security and then try the gear in my home. If you like the preamp and speakers, then at the very least, you need to keep those variables stable. Keeping an open mind is fine if you want to find yourself talking about a much more expansive purpose, but otherwise, you want to at least have a portion of your listening experience be of the the single new piece of gear, right? Nothing to prevent you from moving on after that, but you'd have a baseline you wouldn't otherwise had.

This is expensive stuff and there'd be less reason to second guess yourself if you put the effort in on the front end.
Sounds like he’s really pushing the Einstein integrated which might be the way to go. I’d ask to take it home that way he’s not influencing you , you have more than an afternoon to hear it in your system in your own home which I bet sounds better than a room or big open space at a store. If you live too far away then that might not be an option. Definitely best to try it at home if you can. Depending on your relationship with the store they might not even ask for a card if they know and trust you... the two brick and mortar stores near me let me take gear home often without any deposit. Sure going to miss the small boutique stores after Amazon owns everything.
He's not a fan of PL, but is happy to work with me.
Bring my preamp or not?

I wouldn't expect a dealership to be a fan of a line they don't sell. Even leaving the economics aside, they may not have heard many of them.

At your level of audio system, you need to hear the 4B3 with the gear it'd be used with. No question about that. Whether that's at your own home or at the dealership first depends on the dealership's policies and your relationship with them.

Any sale should be contingent on extended home audition. If the dealer resists, remind them that Audio Advisor will give you 30 days to decide. Locals should be able to do as much for you as the mail-order guys, for heaven's sake.
Thanks for the input, keep the comments coming. Audio Advisor will take the return like you said, but if I did return the amp it will cost a couple of hundred dollars to do so. Yup, I know, it's the cost of making a decision I will live with for a long time.
Bring your preamp, and your cables, and maybe even your source if possible and if the dealer doesn't carry it.

If you like it, insist on at least a weekend demo with the store demo amp (already broken in).  Offer to have the dealer put a hold on the funds on your credit card,  pretty standard procedure for this sort of thing.

You need to be able to hear it in your room if you're dropping that kind of money.
Agree with Vinylzone, if the dealer doesn't work with you walk and buy online with return rights. 
Bottom line, the fewer variables in your audition, the better.  If you can't audition it in your own home with your own preamp, speakers, and other peripherals in your own room, you're not going to be able to draw a reliable conclusion about how it sounds versus your existing amp. 

Bringing your preamp to his place will be better than listening to it with his gear, but it's still not enough info to make a solid decision about buying it. Not when we're in this price range, anyway. 
You should take the preamp home for a few days at least. No question. The real understanding of a piece of gear is listening the music after you stop listening to the gear. Then your subconscious forms an impression of the gestalt and you figure out if it moved your system forward or just made it different. .

My audio guy delivers and hooks the equipment up.
Problem is that he’s over an hour away in NYC, And because of my eye sight I can’t just jump in a car and return something- I can't drive anymore, I'm pushing my wife to take me to the city tomorrow. I don’t want to do it, but if I he won’t let me demo it at home I’ll have to be dastardly and buy it online from Audio Advisor or Music Direct if I like what I hear in the showroom. I really don’t like doing that to a dealer, but I’ll try to get him to put a hold on one of my cards so that I can do as you suggest and try it at home. HR from S-phile wrote that my preamp is very close to the Rogue RP-7 preamp, which he sells. He wasn’t crazy about the idea of having me bring my preamp because he has to move equipment and re-route cables, so there is no way he’ll say okay to me bringing in my cables.
Where do you live? Talk to John at audio connection in Verona NJ  I think he carries Bryston.
By all means take your PL.  Also, Try and determine how many hours he has on his 4B3 in the shop.  Bryston puts 100 on them before they leave. I purchased my 4B3 with 50 hours on it as a demo unit from a dealer here in NC.  It seemed to fully break in after I put another 100 on it. Hearing it with your pre before you start spending more hard earned money is a sensible thing to do.  Maybe even bring a couple of sets of tubes to roll?
KenRad VT231(6SN7)  & Melz 1578s did the trick for me.  Please post your findings after...
I just found Audio Connection on the web, they are, according to Google maps, 1 minute farther away than the dealer in NYC. Has he let you take home demo units?
They offered to lend me a preamp for a weekend when I picked up the Maggies I purchased there. I haven't taken them it yet as I wanted to wait for the Maggie's to break in.  I'll be talking to John soon about preamps.
I can't do a weekend loan, my traveling situation demands a week. I see he has a 30 day return policy.
Everything I've heard about John at Audio Connection makes me believe that if I lived in the area (I used to) I'd want to start a relationship with that store. Not only for this purchase, but for future purchases and for his advice and support of the gear. I have bought locally and I'm not crazy about my local store; they don't get back to me, they seem to have other fish to fry (bigger clients). They don't understand that this is a longer term relationship. 
I heard from the dealer I was going to in NYC. He doesn’t offer demos. Purchase, 30 days, store credit. I’m calling Audio Connection in an hour. My only concern with Audio Connection is that he will take a Bryston return within 30 days. I’ll find out soon enough. I wish there was a Pass Labs dealer somewhere near, I’d love to hear/try a X250.8 amp. I'd have to order from Overture AV in DE and take my wife away for the weekend if I had to return it. They do have nice gardens in Wilmington, so she might like that. Seriously, I just want a great amp I never feel the need to return.
"He's not a fan of PL, but is happy to work with me.

Why would he be a fan, he's not a dealer. He wants YOU to try something he has, naturally.

A good dealer wouldn't say that. He would just be cordial, and encourage you to bring it. Fail.
On my way to Audio Connection to get a demo!
New amp will mean new speaker cables and interconnects between the amp and prreamp, power cord, more helping for the economy.
I’ll keep you posted.
Trying to demo a component can be almost impossible. I have experienced same as you. I think that is why we have so much new stuff on second hand sale. 
after demoing the Bryston ask to demo the Big Belles amp and Belles preamp

 Did they give you a demo to take back to your home or did you take your equipment in? 

I have bought from John...  he is a character but I think he is passionate about audio for sure 

I ordered something he didn’t stock, he was the closest dealer but he was confident I would like it and said I could return it if I didn’t 

I thought that was fair.     Turned out to be one of the best pieces I’ve ever owned.  Talk to him he took good care of me , I think mainly because I talked to him and he is a no BS guy....  didn’t try to upsell or steer me to something else.   Give him a call 
Sorry, re read and saw you were going to Audio Connection 

keep us posted 
Why not but you will like a solid state model better on a bryston amp especially with Harbeth speakers since they are accurate and so vivid on a solid state system.
I’m thoroughly enjoying the Bryston John was good enough to lend me. It’s been powered up since Sunday, and I’ve been playing vinyl, CDs, and my TV through it 24/7 and have been enjoy the sonic improvements along the way. Overnight I put on a CD and put the player on repeat. John is a great guy. I highly recommend him.
That's the way to do it, audition at home with your own equipment.  Strangely enough when I purchased, my room deficiencies were "exposed" and soon found myself working on room treatment.  My older odyssey was by no means a bad unit, it just seemed to be a bit of a blanket tossed over my Maggies in comparison.  Everyone has their own flavor they love but I particularly like tubes in front of the 4B3 with the planars.  Cheers.