Difference between Luxman L-550A II & L-507u

I'm a Luxman dealer so take it for what it's worth. I'd forget about the class A vs AB. I'm still struggling to tell which models I prefer. Hearing class A many think that means better and it may in many cases but the Lux AB stuff is equally good to A just a bit different. Without getting to audiophile. The Class A Lux is what you would expect. Bit more romantic. Lush etc. Warm without being too warm. The AB hits harder and is more dynamic. But sitting in front of both, you'd be hard pressed to pick one type. They are both quite amazing. For example, I prefer the AB stuff on a large speaker such as the Harbeth 40.1 and the Class A on say an HL5. Just for sheer control and authority. Both are equally refined. Bottom line, you pay big bucks for small gains in audio Is the 507 worth the extra over the 550. In many instances I would say yes. The 507 is more detailed and refined as it's a more exact piece. I could live with either.

Thank you for the detailed answer. Since you are around both pieces a lot, could you tell me about the heat output? The 550 ran pretty warm, but thankfully not as hot as the 590. How is the 507 in terms of heat? Also, any feeling about the onboard phono stage?

Harbeth has been on of the brands I"m looking at (probably HL5) and Devore (gibbon super 8 or Nines). Thoughts with either Luxman?