Newbie - Which USED Amp & Preamp for under $2500

If you are definitely planning on upgrading your speakers in the near future, I recommend that you not upgrade your preamp and amp now. (I realize that this is not what you want to hear, but IMHO, that is the best way for you to approach upgrading your system.)

Pick out the speakers that you want, and then match your future power amp, (or integrated amp), to your speakers.

The amp to speaker synergy is very important, and to pick your amp first, will limit your selection of speakers that will sound good with your amps.
(i.e. as examples; a power hungry speaker will not work with a low powered amp, or a speaker that has a very low impedance, may severely tax the power of an amp that does not double in power as the impedance is halved.)

My two cents worth anyway.

Used Pass Aleph gear. Aleph P preamp for under $1000 and Aleph 3 for under $1000, Aleph 5 for $1400, or Aleph 2 for around $2K. I had all of these and for the price, they are excellent. You could also look at used FirstWatt, though you would have less power.

Like Kurt_tank, I too am wondering how attached you are to your speakers. With $2500 to spend on amplification, you could easily outclass the 602s. I think Kurt is giving you advise worthy of consideration.

I have a Conrad Johnson MF 2250 in my "2nd system", along with a C-J PF-R SS Preamp. Paid about $2K for the pair a few years ago, love them.

I had a McCormack DNA .5 for a few days once. The seller had shipped it so poorly that the entire metal case was bent, & twisted. It not only worked (!), but sounded very nice.

An MF2500, or DNA 1, would be other SS ideas......

Kurt_tank advice is priceless. If you are going to invest in your system, settle on the speakers first or you will start going around in circles; the speakers will then inform your amplifier options. I think your current speakers are not suitable for tube amps in your price range, they really are best suited for SS.