Best Tube preamplifier under $600

I second the motion for a Conrad-Johnson PV10 (specifically, the line stage / no-phono version, the PV-10L or PV-10AL).

Alternatively, given that you plan to buy a tube amp, you might want to combine your amp and preamp budget and invest in a good tube integrated. Keep your eye out for a used PrimaLuna Prologue One or Prologue Two. Google for info and reviews -- very nice amps. Similar (and available used on Audiogon as I write this post) is the Cayin A-50T.

Another integrated option -- the Unison Unico -- has tube preamp stage and solid state amp -- sounds warm and musical.

Finally, you might again combine amp and preamp budget and look into the integrated amps from Jolida. You could get a new or like-new one from Underwood Wally, who advertises constantly on Audiogon.

Thank you all for the suggestions, very helpful! Anymore recommendations are definitely welcome!!

Question about power rating of tube amps:
While shopping around ive noticed a lot of tube amps typically have power ratings that seem incredibly low (like the Cayin A-50T at 35 watts). I know wattage is the least important factor to consider if I'm looking for pure sounding tube components, but is that wattage high enough to drive my front speakers, or is there an optimum set of power conditions I should look for in tube components that would specifically complement my 2 front channels?? I've listed their specs below.

Thanks again!!

Klipsch RP-5 (currently driven by B&K reference 125.2) Specs:

BUILT FROM: 1999
BUILT UNTIL: 2001
DIMENSIONS: 42.5" (107.9cm) x 9" (22.9cm) x 20.3" (51.6cm)
ENCLOSURE MATERIAL: Medium density fiberboard construction (MDF)
ENCLOSURE TYPE: Sealed (HF/MF drivers) / Bass reflex via rear-mounted port (subwoofer)
FINISHES: Black ash vinyl veneer
FREQUENCY RESPONSE: 25Hz-20kHz(+-)3dB
HIGH FREQUENCY HORN: 90(o)x60(o) Tractrix(r) Horn
MIDRANGE: K-1084-S 8" (20.32cm) Aluminum cone / cast polymer frame
NOMINAL IMPEDANCE: 8 ohms
POWER HANDLING: 150 watts maximum continuous (400 watts peak)
SENSITIVITY: 96dB @ 1watt/1meter
SUBWOOFER: K-1087-K 12" (30.48cm) Fiber-composite cone
TWEETER: K-105-K 1" (2.54cm) Titanium dome compression driver
WEIGHT: 72 lbs. (32.7kg)

Your Klipsh speakers should be very easy for a tube amp (even a modestly-powered one) to drive. They tend to be very efficient and an easy load.

Jhold is right on the money with his suggestion of going with a tube integrated. Something like a Jolida 302b should be roughly within your price range, although there are several other well-regarded tube integrated amps under the $1000 price point.

Your speakers make it relatively easy to have great sound from a budget tube integrated amp. My advice is to take advantage of it and go that route.

Michael