I agree it's tough to say. There are some very nice integrated amps out there, and I don't think you lose much, but there seems to still be a little something extra that comes from separates. Melody's new upscale integrateds, the AN211 and AN300B have absolute top notch parts in them. (Mundorf silver-in-oil and Jensen copper foil paper-in-oil caps, hand wound interstage transformers, etc.) However, there is still a certain magic that the Pure Black 101 and either M845 or PM845 monos has that the former can't seem to match. The trouble is that it's not apples to apples. Maybe I just like the sound of 845's better than 300b's? Maybe one is a slightly better design than the other? Maybe it's just that the Melody preamp is just that awesome? Who knows?
A better comparison would to look at a pair of Melody monos next to an integrated with the same tube and design. They make mono versions of the Astro Black 50, Astro Black 22, Astro Black 8, and Astro Black 40, plus a couple of other variants, but we aren't importing them right now because there are just so many models. Maybe someday if there is demand that will happen and we can do a cool side-by-side test.
All of that is besides the point, however, because objectively there ARE very good sounding integrated amps to be had. Buying a separate preamp and monos will always be more expensive, at least when you're comparing a similar level of performance. If you don't plan on mixing and matching in the future I think an integrated amp (tube or SS) can represent a great value. Just like anything else in this hobby, you have to pick your price range and then decide whether you want to eek the last bit of performance out or go for value and simplicty.