You can turn the fans down. There is a 4 way fan speed switch inside. Factory setting is maximum speed, but if you do not plan to put your amp in a cabinet of some kind, you can safely turn it down to minimum speed, or disconnect it altogether (that is how I used to run my Ref110).
Comparing the Ref110 vs Ref150, the new amp has double the energy starage (24x 470uf/450v caps in Ref150 vs 12 x 470uf/450v caps in Ref110), different coupling caps (may be the same as those used in Ref 40 - but not sure) and KT120 tubes.
I haven't had a chance to hear the Ref110 vs Ref150 side by side, since my Ref110 is long gone, but I think that the Ref150 has better bass (although still not state of the art in solid state terms), more clarity and resolution on top. It is increadibly dynamic, and retains one tf the Ref110 halmarks - excellent PRaT and midrange texture.
Overall, I still prefered my Dart, but the sound quality difference was nowhere near as big as the price difference may sugggest ($30.000 vs $12.000). If I hadn't had the dart, I might have bought the Ref 150.