First: If you need an amp and preamp for $600 then an integrated is best. Second: Your budget calls for mid-fi speakers, so separates are not going to make much of a difference, if any. So an integrated is the way to go. A preamp/amp would also require more $$ for cables.
Considering your $600 budget, apartment space, and listening taste, I would say the Audio Refinement Complete integrated amp is the way to go. The AR Complete also has the clarity and dynamics for better speakers, so it affords you the ability to upgrade speakers later down the road.
Are you looking for an integrated or just an amp? Do you already have the preamp? I've heard the 602s in a store and have heard a few different Paradigm monitors, but never the v20s. I understand the v20s are very nice. From my brief experience, the 602s and the v20s will have a very different sound. I thought the 602s were thin and bright, and the Paradigms I heard were on the dark side. These are characteristics to consider when choosing an amp.
You might consider the Creek 4330 integrated or some of the Jolida tube integrateds. Also look into the Rega Mira. Have you auditioned any amps in stores or have an idea of what you like? If you only need an amp do some research on older Bryston 3Bs or the Anthem Amp 1. The research is part of the fun!
my vote is the anthem amp 1. you might want to look at some spendor speakers; our musical tastes are close and these imo a better musical sound.
a tube integrated might be the way to go; vtl; arc or rogue would do nicely....a little higher than your amp budget but if you need a pre...it might be the way to go.
I "downsized" one of my systems to an Audio Refinement Complete (ARC) driving B&W DM602s2. I preferred the B&W's over the Paradigms. While the ARC will work with the B&W's I would not recommend this combo unless you have a good, smooth front end as it is capable of revealing any problems. If you want a more forgiving sound, try an NAD integrated amp.
I am currently using paradigm studio 20's with an anthem integrated. Very nice. I personally don't care for the B&W line until you get up to the CDM series. I owned the next step down from that (CM) series and didn't like it. I've also spent time with the 602's. I can't pinpoint exactly what I didn't like. Just that the paradigms had more of what I like. I was using parasound amps for a while, but switched to the anthem integrated and am much more happy with that.
Fullrun, you might want to take a look at a Sonic Frontiers Anthem Integrated 2 that is up for sale on Audiogon. The seller wants $650 for it. To be honest, I don't know a thing about this integrated, but based on the good reviews by forum members of Sonic Frontiers/Anthem products, it may be worth it to research this piece. Good luck!
I can't help much with your speaker selection, but can personally recommend the AudioSource Amp 1/A (80 watts/channel and an excellent performer) as well as the AudioSource Pre 1A preamp highly. Both the amp and preamp can be purchased new for around $500 - $550 total. They are solid entry level pieces, and the amp is stable into even 2 ohm loads.
If you feel you want even more power, the AudioSource Amp 3 (150 watts/channel at about $550 - $575) might be a good choice. See the following link for pricing, additional info, and a dealer who sells these units:
A used Audio Refinement Complete integrated amp.
Had the 602's with a Musical Fidelity A3 and the combo was quite good! The 602's are quite good for the price and the tweeter is very resolving. Audition as many speakers as you can and go from there.
In your price range a used Audio Refinement Complete is very nice. The price maybe to high but a used Audio Analogue Puccini is also sweet. NAD is also decent but tends to be a little forward to my ears. If you do not want used you may also want to audition a Jolida tubed or hybrid integrated.
Cambridge Audio from England makes a great integrated amp A500 which carries a Stereophile Class C rating. It retails for $450. You can pick up a dempo from Audio Advisor for $339
I prefer the Paradigms over the B&Ws, but I'm buying the 602 S3s because I can get them new in Europe while on vacation next month for get this, $452! The slight difference between the Paradigms and the B&Ws simply isn't worth $200 to me. Check out this link: http://www.hifissimo.com/store/search.cgi?user_id=&database=base.db&template=liste.tpml&2_option=2&2=b%20et%20w&output_number=4
And that's before you get the taxes back (should be around 10% I forget the the exact rate of the VAT.) The Paradigms have a clearer high end, pure more, really noticable on cymbals. If you listen to jazz, buy those.
As for amps, I really didn't like the Cambridge A500 - I listened to it for about one minute before moving on to NAD. Sounded no different than a good receiver to me, and I wanted more fidelity.
It really depends on the budget - if you have no budget, go for NAD. If you can go to $700, the Rotel 1060 is clearer and has lower distortion levels. If you go to $800-900, you have a pick of excellent sounding pieces like the Arcam 750 (incredible soundstage - what I'm probably going to get,) the Rega Mira...Note I am noting low-wattage amps because you really don't need the power for the 602s or the Paradigms, they're pretty sensitive.
Given your preferences in music and sound characteristics you should consider looking at Triangle speakers. The Zephyr IIs may be a bit more expensive than the B&Ws or Paradigms, but a much better and more refined speaker overall in my opinion. And the bonus, they're extremely efficient so you can pair them with flea-powered amps and still get a ton o' sound.
I'd look at Cambridge and Creek integrateds. Best of luck.