I wouldn't call 2-in-1 combos inherently bad. Most times when you have a device that has to do more than one dedicated thing, you'll have compromises. Basically combo units are DAC and amp sections that are put in the same box together, which solves issues of space and convenience, but creates new ones like potential digital noise issues. Also DACs tend to be upgraded and improved constantly, so with an all-in-one unit a perfectly good amp section could end up "obsolete" in a year because of the aging DAC attached to it.
I'd personally stay away from Fostex's A8 combo that was marketed alongside the TH900. If I were doing a combo unit I'd probably look at the Woo first or maybe the Burson Conductor (which costs more, however).
As for solid state being more suitable for low impedance headphones, this is not necessarily true. Again it really depends on the specific design of the amp. The all tube Zana Deux SE is actually one of the quietest amps I've ever used. Ever. Absolutely black background. The solid state SPL Phonitor however hums rather noticeably with sensitive headphones. Same goes for their sound signatures: people tend to associate tubes with being inherently warm and bloomy, but this isn't true.
I don't have that kind of disposable income like some of the other members on here, but I do like my amp and DAC to last a long time without breaking down. Looking at the combo units you suggested, it is either the Woo Audio WA7 Fireflies or Burson Conductor. If I do go for separate units, I go with the Burson Soloist as my headphone amp and the Resoessence Concero as my DAC.
Thanks for debunking this myth. Looks like I still need to do more research and explore my options then. Wish I could do an audio meet to try out headphones, amps, and DACs to get the feeling out of them.