I've tried a few, so I guess I can recommend some stuff.
Disclaimer: all the pickup/preamp swaps I did, I did because I wanted to get closer to the vintage pre-eb Stingray tone ('70s Stingrays)
I had a USA SUB, 2 band preamp, which is basically your Stingray Classic without the pretty finish and wood. I put an Aguilar AG 4M pickup in it - it's a great pickup, the tone was a bit more rounded and had nice mids to it, a bit fuller, great for everything.
Now I have a 3 band Stingray, and I just swapped out the preamp and pickup this weekend. I wanted to get the 2 band tone back, because I prefer it, and, again, to get close to the pre-eb vintage Stingray tone.
I've put in a John East MM preamp (it's basically a 2 band vintage ray preamp + mids control) - absolutely fantastic sounding preamp and very versatile with the mids and sweep controls (you can even make a wah effect with the sweep knob). Really, really great, and John is a stand up guy. It's based on a '76 Stingray preamp.
I've swapped the pickup for a Nordstrand MM4.2, which is a replica of a '78 Stingray pickup. This thing sounds really really nice. It's nice and round, warm, smoother than the stock pup and no clank. In combination with the East preamp, my Stingray really sounds like the old classic Stingrays with an additional mids control, very versatile. Definitely my favorite combo, and it just NAILS the vintage ray vibe.
For less expensive pups/preamps, there are some nice options as well. Seymour Duncan is a popular option, and there are less costly 2 band Stingray preamp clones (Retrovibe Stinger, MojoTone).
