Thinking about upgrading, but wondering about diminishing returns

I have mostly Conrad Johnson Gear-- LP260M SE mono blocks, with 8 kt120 tubes each.  These are SET amps that put out 270 wpc.  I have the Conrad Johnson  ET5 preamp, with a Telefunken 6922 tube, and a pair of EgglestonWorks Viginti speakers (their new take on the Andra's).  Phono stage is the Pass Labs XP-15.  For source material I have a Rega  RP10 with the Apheta2 cartridge, and the Oppo BDP105 (Modwright upgrades) CD player.  I have Audioquest Colorado interconnects (go easy here :) ) and Audioquest Oaks for speaker cable.  My listening room is 13 x 19, carpeted floor, drywall walls and ceiling (well insulated).  I set up with the equipment on the long wall, with me listening from the 13' side (with speakers in about 2 feet and about 8.5 feet apart).

I listen to a mix of vinyl and cd's; vinyl when I have several hours to listen quietly (and clean the records on the Clear Audio Matrix Pro Record Cleaning Machine).  I listen to CD's when I have less than an hour, and for when I work out.

I am thinking about upgrading a single component, probably my preamplifier, to a Conrad Johnson GAT2 or a Pass Labs XS preamp (solid state). My thought was that this will likely make the most improvement in soundstage, imaging and overall musicality.  A used GAT2 will be at least $14K and the Pass XS used is about $19K.   I am interested in hearing from those who have had experience with upgrades like this.  I am also wondering (as a middle class guy)  how close does this come to the law of diminishing returns?  Finally, is this the component you would upgrade, and if so, if you had a wide range of musical tastes-- mostly classical/full orchestra in vinyl and rock on CD's, which preamp would you select?

Please feel free to take a swipe at these questions, and also feel free to point out if you feel I should be asking different questions that will help increase my knowledge and advance the cause for others.

I am likely to go to an audio show sometime, but please don't suggest that I go and listen to the gear, as that's at least 200 - 400 miles, and I don't feel right about going to a brick and mortar store knowing that I will almost surely buy used.

Thanks in advance for whatever assistance you can provide.