Autre, like you I owned the GCD-575. Last year, after many moves, the transport finally gave out. I listened to the Rega and loved it, but it was also in a much higher end system than I owned, and even the used ones were more than I wanted to spend. The MH 25 and the Onix are identical and I was able to buy the Onix new for half the cost of the MH. It gives up only a little to the Rega and fits my needs perfectly, as CDs are secondary to vinyl for me. When I get the bug, the Onix is very tweakable, and can be greatly improved with a little work.
Viridian and Italian are correct about the quirks of the Onix transport - it can be pretty slow. However, it will read anything I put in it, which more than I can say for the Adcom.
Whatever you decide, you'll be amazed at the improvement you'll hear. The GCD was a nice piece in its day, but the Onix or any of your other choices will be a big upgrade.
Viridian and Italian are correct about the quirks of the Onix transport - it can be pretty slow. However, it will read anything I put in it, which more than I can say for the Adcom.
Whatever you decide, you'll be amazed at the improvement you'll hear. The GCD was a nice piece in its day, but the Onix or any of your other choices will be a big upgrade.