Not really but all the Theta amps I've heard have a slightly forward and lively sound. Not heard them on Aerials.
After using the Dreadnaught on and off for a few years, I can say I would have never called them bright. It is, if anything, slightly laid back. (It sounds closed in until they warm up a while.) I found it to deliver sound in a very convincing way once warmed up. My major complaint was a little conjestion in the mids compared to my Ayre V5xe.
I would look at associated components including cables. You can't fault an amp for delivering what it is fed (Which I think is the case.) I have never owned the Model 8's but have heard them numerous times and I always considered them a touch bright to begin with. A neutral system is going to reveal some bright recordings because they are a bunch out there. My point is, in your system, it may be bright but I really don't think it is the amps fault.
I run a Dread I and Aerial 10-T's (newer version). I've had them a couple for a couple of months now and wouldn't say they are bright, not, that is, in comparison to other systems I've had. I would say that they are extended but real.
For example, now I can hear that little hard metal ting that you're supposed to get on triangles or cymbals. On bright systems this didn't sound realistic. The Aerials sit back a little; brighter systems jump forward at me. I've listened for 3 hours straight and was cool with it;. bright stuff runs me out after about an hour.
I use Audience Aud 24's IC, Cardas GR IC, and Audience AU24 speaker cables.
I have used a Dread in the past with B&W speakers and agree that the amp has a somewhat laid back character and would not characterize the amp as being bright in any way. However, the Dread is revealing and if your cables tend towards the bright side (e.g., Nordost Red Dawn) the amp will reflect that. Try Synergistic Res. Ref. X2 cables. Just a thougth. Good luck!
You don't mention if it is a Dreadnaught or a Dreadnaught II. I had a Dreadnaught for years and then brought home a Dread II for a possible upgrade. It went right back to the dealer. It wasn't as musical as the original Dreadnaught and I would consider it bright, almost as bright as the Krell amps were in the 90's.
One caution I would like to raise is that a true authority on amplifiers has recently asserted to me that he can demonstrate how big solid state amps turn bright when they are current starved.
If the IC recommendations do not pan out for you, try looking at whether you are not providing the amps with enough good clean power.