Why are these speakers so bright but others not?

You are correct about different amps for different speakers - the match is critical, but even more critical is the speaker set up in the room. And, even if properly set up I would not expect them to sound the same, no matter how highly regarded. Except for a slightly low ceiling you have a reasonably sized room for all of these speakers. Different designs require different speaker and listening position placement and acoustical treatment to get the best sound. You are presently considering 3 different designs all of which need different placement considerations. You have a lot of work to do on each speaker before you can fairly judge its value to you in your room.

the psb's are by far the best match with your system. the ocm is avery neutral ss amp and the arcam is likewise. the psb's easily are the best overall performers in this group.

I did play with speaker and seating placement somewhat, but I was just surprised how not good I found the Revel and 3.6R's in my room. Not that they aren't excellent speakers - the clarity and lack of distortian in Revel's midrange came through unhampered as did the sense of space with the Maggies (although obviously constrained in my room and placement).

The PSBs and the Martin Logan's ended up being located very close to the same location which was interesting because they are so different a design. My PSBs (original golds) are front ported and the MLs have the panel and on of it's woofs rear firing. I think the MLs also did well in my room because of the narrow dispersion (i.e. mitigating brightness of untreated room). I thought I could place the Revel's in a fairly similar location to the PSBs, but I could't get enough bass without putting them pretty close to the front wall. I do have lumpy response in my room below 60Hz.

I originally built my system around the PSBs way back when, hence the amp, but I fell in love with vocals and instruments on the MLs despite some of the other tradeoffs (bass in my room, some dynamics). Midrange is so much cleaner and so much more detailed.

It could be a number of things, but since I have had a similiar situation recently, I wanted to let you know that uncovered windows (at least in my room) were a disaster.

My system was sounding bright, and I recently bought auralex room treatment. I put it on all of the critical spots; my system was still harsh sounding. Then I saw a picture of someone's room on this forum who had a similiar room setup and size (smaller than your) and bingo! I had blinds over my windows and assumed that they would defuse the sound WRONG! Just like this gentleman, I put a folded blanket over the blinds and added some other treatment to just the window. I got the soft warm fuzzy sound back instead of the "I paid all this money for this kind of sound what the?"

Can't promise you it will work and I feel your pain, but it just takes a minute to listen to it before and after you throw a comforter over the window to see if it helps.