Monitor speakers for Tube integrated?

Reference 3A De Capo i's, oh forget that, I just saw your price limit.

What about headphones?

ARS Pieces?

Triangle Titus 202 might be had used for close to that. I think these would work very well with a tube amp.

Scrape up just a bit more and snag the pair of Green Mountain Audio Europas that are available on the 'gon.

You may be able to use a wide variety of speakers. The variables are how loud, how big the room, how close are you to the speakers, what kind of music will you listen to, will you likely upgrade in the near future. As an example, I am playing CDs through a Sonic Impact T-amp, 6 W/ch, through Dynaudio Audience 70 floorstanders in my bedroom. They are 87 dB into 4 Ohms. I can achieve loudness of 80 dB with little distortion in a room that is 17' x 13' with vaulted ceilings playing Classical, Jazz and Folk. So, it depends. But I would say go for the highest quality speaker you can afford and settle for less volume with higher fidelity. You can never go wrong with fine speakers. They last the longest of any component you will buy. Good luck in your quest.