I have a SR 300. The pickups aren't that bad, not the greatest, but workable. I thought I would have to replace the pickups first thing, but after a bit of testing they weren't that bad; my band actually liked the sound. The preamp, however, is HORRIBLE. IMHO you may be best off adding a bart or aguilar preamp and seeing if you like it.
The pickups are a MK1 rout. Not many pickups will fit here without routing.
There are these from Bartolini:
Direct retrofit for MK
http://www.bestbassgear.com/bartolini-mk5-pickups.htm
The P2 shape will also work but are a bit narrow (1.25 vs 1.33; the difference won't affect the tone)
http://www.bestbassgear.com/bartolini-p25-pickups.htm
From Nordstrand:
Any of the 5 string soapbars also come in a P2 shape. The Dual coils, big splits, big singles, and fat stacks should work.
http://www.bestbassgear.com/nordstrand-pickups.htm
I don't know of anything else that will fit in your rout, and none of these are active pickups.