From everything I've experienced and read (I haven't owned one, but have played through a couple). They are rather efficient, like 103db 1w/1m so they get loud quick. Not to much on the low end because of the small cab size (1.8 cu ft) but is able to get good lows with a modest tweak in the eq. Great mid range presence, over-all a pretty sweet cab... I had a yearn for them not to long ago ( I still do, but am going a different way). Another plus is that they are relatively light weight (44lbs).
$300 is a good price, they tend to go for around $300 to $350 shipped on the boards.
I have no idea how it'd pair with the Kappa Pro Avatar and the SWR, my only concern would be the sensativity of the Avatar. Other then that I'm sure it'd sound great.
If you're looking for gobs of extra low end extention then I would look else where. But if you want a nice loud, punchy 15 then this might be where you want to look.
Peace
-Benny