Best bookshelf monitor under $1000 new or used.

Are there any good tube amps that would work well with this set up (computer as input and bookshelf speakers as output)? Or is it not even worth it until I get myself better quality stuff?

When you say bookshelf speakers, are you merely referring to the size, because placement is very important? I have a pair of Spendor S3/5 that are very nice, but they wouldn't sound as good in a bookshelf. The Spendors play well about four feet away from the back wall. The Linn Tukan, which was purportedly designed to placed within a few inches of the rear wall, might be a better choice if the speakers were placed near the back wall or in a bookcase.

Well I would prefer them against the wall but it doesn't really make a difference. If I had to I would store them against the wall until I used them. Yes I did mean size when I said "bookshelf"

Spendor S3/5 se's are excellent-to-great speakers, esp. at the used price of around $600-$700 I think. Almost a no-brainer IMO.....

I own a pair of B&W 602 S3 speakers. These can be purchased used for about 600.00 Oddly enough that's what they also cost new. There's a reason. They're a great value and they are a nice bang for the buck. Most other speakers brands devaluate by a third in a matter of months. They sound great (for 600.00) and don't need much power as long as it's quality. I'm told these were the last of their breed to be built in England. Frankly, to me, they sound a lot better than their current replacement in the B&W line-up. I use them as rear surround speakers now but when they were my mains I enjoyed them every day. The good news is that there are loads of great speakers in your price range. My last suggestion is that you buy something that's used and in great condition. Quality well cared for used gear is as good as it is when it's new except that it's already broken and except for the 602's, usually a lot less money.
Good luck. ps If you go this route, stick with the S3 version.
Mike