What about a used Peachtree Audio Decco? Should be around $500 and it meets your stated specs.
Re the possibility you mentioned of a tube integrated, keep in mind that a tube power amplifier (or the power amplifier section of a tube integrated amplifier) should NEVER be operated without a speaker load (or equivalent, in the form of a high power non-inductive resistor). That is particularly true if a music signal is going through the amplifier.
If you do want to consider a tube integrated amplifier, I would suggest choosing one that has both preamp out and power amp in jacks, so that you could operate it with no signal going into the power amp section. You could then attach a relatively low power resistor like this one to each of the amplifier's two output channels, as a dummy load.
I'd second the Brio 3 as having great bang-for-buck at that price. Shameless plug - If you want more power, a definitive step-up in sound, a remote and phono-out and preamp-out (you could feed your current amp with it if you wanted to using it as a preamp only), get the big brother to the Brio3: the Rega Mira 3 - I will be putting a minty one on the market this week at slightly more than your price range. There is also one currently on A'gon (no knowledge of the seller). I had both Brio3 and Mira3 in my office system and can tell you the difference is significant, though both are good performers at their pricepoints. Brio 3 is 49 watts and the Mira 3 is 60 watts. The Mira 3 offered greater clarity and midrange finesse in my system. Slightly better bass performance. Better separation / soundstaging, and was simply more engaging to me overall (which is why I chose it). That would make a great office system amp. Tubes should offer a few options. One I particularly liked was a Sophia SET Baby, but it only puts out about 15watts. There should be something by Cayin, Jolida or Prima Luna that may fit the bill.
I'd second the recommendation of the Peachtree Decco. I'm using one as a preamp and DAC with a tube power amp in the winter and as a stand-alone integrated in the couple of summer months when the class A tube amp puts out too much heat in an un-airconditioned house.
The versatility of the multiple inputs for the DAC, including USB, make it a good candidate for a later office system, as well.