A good combined amp/dac can be quite good, but depends on the product of course. The K5 Pro served me well, but it was a much better amp than it was a DAC haha. Definitely benefited from a better standalone DAC later, but I don't regret it as an entry-level unit for my HD 650. A combined unit is mostly for convenience and might be cheaper than separate units.

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>would you say r/AVexchange is a good place to look for used ones to pick up ?

Not sure, never visited that sub. I usually buy my audio stuff from Amazon or another site with a good return policy, in case I don't like it or it doesn't work as well as I thought it would. Have found good deals before on refurbished units. You could check a manufacturer's website for b-stock (items others bought but returned), for example.