From the HUs you mentioned, they seems to be compromises no matter which one you take.
From my point of view, starting with your Eclipse 8051.
This unit was alredy old skool from the day it was introduced. But what it got so right was the analogue section of the unit. Most well done was the power supply and the highish pre-out, if your amps can take it. Smooth overall sound but nothing really to get excited about as well.
Alpine 7998R, like you I've always drew over it when it was new but considered way of of budget for me. It starts what some veterans would call the newer Alpine sound compared to past legends like the 7909,7939 and 7949. It's weakest piont like all recent new Alpine units is the power supply which is of the switching variety, DC-DC copnverter aside. The sound is somewhat less full bodied compared to old skool Alpines due to poor output, I've scoped it to outputing at 4V but at cliping point!, actually it started to clip at 3.9V. But makes up for it with very versatile built in tuning devices, like T/A, EQ, X-over amongst other things, if you are a believer of such things at all?
Clarion HX D1 / 9255EXL, are decent players in their own right, the sound is highly neutral(the 9255EXL is a little warmer). I personally found the HXD1 neutral to the point of being sterile, but the 9255 is more musical overall. Both units have very strong power supplies and I have scoped both to measure 8V output cleanly! Both units are somewhat let down by chossy disc mechanism. It even rejects reasonable condition ORI CDs! Between the two, my pick is the 9255EXL.
Sony C90 + XDP4000, Heard both this units breifly, and sound nice and smooth enough. It was done at a time when the brand Sony actually has credibilty! Those were the days! It was also one of the units I had lusted over until I heard the XES system! But I can't say much as I only had brief sessions with it.
So my vote? the Alpine 7998R actually because I eventually bough a pre-owned unit and still using it.
As for your vote, I would suggest you think about the following 2 Questions.
1) Are you building a purist system or following that path somewhat? Are you anti processing of sound?
2) Or do you like to go full active and have the most flexible tuning tools to help you get there?
Both ways also can lead you to a good SQ system. Choose your path now, as you're starting with HU.
