My experience with outboard phono preamps (now called "phono stages") is limited to the Parasound PPH-100 I purchased as the PLD-1100 preamp I had purchased prior didn't have a phono input, and I didn't know that at the time. I'm very satisfied with the overall performance, so I guess that's a "thumb's up" for Parasound.

Insofar as you not being particularly pleased with the phono stage of your Adcom GP-565, that raises a flag. I own one, but use it for a 2-channel system my HDTV is connected to. When it was new and tested by Stereophile, the mag compared its phono section to an Audio Research preamp costing 10 times that of the Adcom, and actually preferred the Adcom's phono stage.

So, as you've already posted a problem with an annoying hiss coming from the preamp, it may be that the entire unit is faulty in one way or another. If the phono stage is working properly, you might be surprised that the addition of an outboard phono stage may not give you the improvement you're looking for. Then again, the GFP-565 is about 15 years old, so there may be a huge difference. I'll be interested to hear which unit you buy, and what your observations are.