best monitors above 300Hz

What are the best monitors in the frequency range above 300Hz? I'm using a Tact room correction system. It includes an active crossover. The manufacturer recommends placing a pair of very fast subwoofers in the corners and crossing over to the main speakers at around 300Hz.

My ideal price-range is below $2000 new or used. So far I'm considering using VMPS 626R's and bypassing the woofer (it's crossed over to the midrange ribbon below 300Hz).
An 8" Lowther in a sealed enclosure would be ideal.

I think the bigger problem is finding a "fast subwoofer." Subwoofer by definition operates below a woofer, and would operate at 40Hz or below. Most/all commercial subwoofers claim to operate from a 100Hz down and that is probably stretching it a bit for music reproduction. You would need a direct firing midbass/bass enclosure to ideally mate with any speaker crossed over at 300Hz. Unfortunately, I do not know of any high quality commercially available midbass/bass enclosures. If you are into DIY there are many enclosures that you could make (check the vintage Altec website for starters/ideas).
horns? like oris kits?
Thanks for the pointer Ultrakaz. I'll look into the Lowthers. I'm not afraid of a little solder and glue but I don't have the equipment (or space) to do much woodworking.

You're right about it being hard to find fast subs. For the hundreds of subs on the market I've only found a handful that have the potential to produce over 300Hz.

- VMPS makes a sub that plays up to 600Hz.
- VBT/TBI makes a sub that some say mates very well with horn speakers and electrostatics and I could probably push up to 300Hz.
- Tact also makes subs specifically for what I'm planning to do.
- Talon's Roc and Thunderbird go upto 500Hz and 700Hz respectively. They claim to have the "fastest" sub available.
- D-Box's Mammouth [sic] is supposed to be very fast and goes up to 500Hz.
- RBH also has a sub that might do the trick

That's what I've found so far. Of those the TBI/VBT and VMPS are very resonably priced (both under $1500 for a pair); the D-Box and Talons are completely out of my price range ($8000ea for a Mammouth, $2700ea for a Roc, and $11000ea for a Thunderbird). The Tact subs are an expensive backup plan (over $2000 for a pair).
Re the monitors: VMPS are very impressive speakers, especially considering the price: fast, detailed, transparent.
ATC SCM 12's. ATC is well known for their accurate midrange and they are not harsh / bright like B&W can be or dark / cold like DynAudio.