I have used the U70 (i.e. Ultravalve in recycled Dynaco 70 chassis) for about a year interchangeably with an Ultra 550. Modulo the power difference, they were as Frank likes to say very very close. For a long time I had persuaded myself that the U70 was the winner, but the summer heat would bring the 550 back into circulation until one day I discovered that I preferred the 550 after all.
Since then, the 550+ to 600R upgrade is significant.
The speakers you use may make a difference, stable friendly impedance curves and all that to aid the all-tube amps. The U70 does a surprisingly excellent job in that regard but of course it is conceding a natural advantage to the SS output stage there.
Nonetheless, I would have unhesitatingly kept my U70 with its charming sound if only the heat issue in the Los Angeles climate had not been there (I like it 68ish temperature wise). I suspect that this would not be a problem if I resided in the beautiful state of Alaska. 
You are correct that heat is not a huge issue here. In fact, except for maybe 10 days during the summer when it approaches 70, the added heat would be welcome!

The Salk Soundscape 8's do have relatively a stable impedance curve, IIRC, though at 4 ohm.
So, what is it you prefer about the 550/600R over the U70/ultravalve other than the reduced heat? And what is it in SQ that makes the U70 have a "charming" sound?
With the AVA 400R/600R you get CLARITY! That is the biggest difference. Life-like sound. Better Bass. Smoother highs. You will notice things that you never heard before.
I've been under the impression that tubes will generally give a smoother top end, possibly with less detail or extension. Your impression is that the hybrid has smoother highs than all tube?
This new amp sounded better with the 8's than the 600R amp at AKFest.
http://www.avahifi.com/index.php?option=com_content&view=article&id=165&Itemid=209
Pete -- really!? Did others share your take on that? Were both amps used wiith the 8's in the same room? What in particular was better about the solid state amp than the hybrid in your opinion?
As an aside, I just have to comment how great these forums are as a resource. I think we can still count SS8 owners on one hand and I'm getting input from two of them! It looks like mine will be counted on the second hand since I've been so slow with the finish decision. I need to get back in touch with Jim about getting things underway again, so I will ask for his recommendations as well.
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As always, it is difficult to assess from verbal descriptions of non-verbal experiences, but I appreciate the efforts. Any opinions about the more abstract experience of engagement with and emotional experience of the music between these two designs (in contrast to particular sound qualities)?
Thanks to everyone - great discussion.
.........Peter