Here's the best thread I could find on the Rocktron:
http://forum.fractalaudio.com/amps-cabs/71223-rocktron-velocity-300-ir-frequency-response-plots.html
The premise of the rocktron is cool, but it doesn't sound like they really pulled it off and instead just added some tone controls. You can dial it in flat though and it seems that a lot of people like having those added controls, so as always ymmv.
To the OP:
My personal suggestion would be something along the lines of the QSC GXD 4, which would be at the top of your budget at $499. It is relatively new on the market, so it has no real track record, but I think Class D power amps with onboard DSP have come of age much like digital modeling. I have the PLD series, which is the GXD's older brother, and am very happy with it. The PLD series has been on the market for about 2 years now and initially had some growing pains, but they've ironed those out and I would have to believe that a lot of the tech in the GXD trickled down from the PLD series.
The reason I suggest something like this is because even though you're sure you want to use a guitar cab today, maybe someday you might want to check out passive studio monitors or something like an Atomic CLR passive. A power amp like the GXD 4 will let you safely power any of those options with it's limiter which is rated for 5W-400W @ 8ohms. Plus it has both twist lock/speakon connectors and speaker binding posts, it was designed for use in both live sound and installation applications, so if someday you want to re-task the power amp you could even use it in your high end home theater for a biamped center speaker with a DSP crossover in the power amp. This power amp will grow with you within reason. It will never have more channels and never have more than 400W a side @ 8 ohms.
Sure, you could use something like the SLA-2 for all of those tasks or a DCM1504L, but the limiter makes it a lot safer if you're trying to power a 25W 8ohm 1x12 guitar cab. Admittedly, the other DSP functionality will probably not be very useful to you, but who knows? Also, the unit is 2U which may be a factor.
Check out this guy's band in a rack which uses the limiter in the PLD to power guitar cabs and a bass cab at the same time
Rig Rundown - Band In A Rack (Continued) - SevenString.org. The same sort of thing can be done with the GXD, of course with 2 fewer channels. The setup is similar to what Periphery are doing with their PLD 4.3.
The iNuke DSP would be very similar to the GXD, but with more wattage per buck.
Edit: Noticed spec sheet for the GXD, damping factor is only 100. This amp was not made for subwoofers, but should be suitable for the guitar's midrange frequencies.