I too use a Basis 2500, with a Vector M3 tonearm, with an ASR Basis Exclusive phono stage, (the best phono stage I've ever heard).
I use a Dynavector XV-1S cartridge.
(Actually it's my second one, as I like it so much that I got a second one.)
To give you some background on my taste in sonics, I'll describe my past cartridges.
When I first got back into vinyl about ten years ago, I started out with a Benz Micro Glider II. It sounded good, (fairly low noise floor, decent bass), but it was a bit analytical for my taste. The imaging and soundstaging was good too.
I then tried a Koetsu Black, and this really opened my eyes to what vinyl could sound like. While the bass was a bit loose, and the treble was not quite as extended as better cartridges, the mid-range was sublime. The imaging and soundstaging was very good as well. In addition, the noise floor was very, very low, resulting in deep, dark, black backgrounds. (This is something that I decided that I really liked in my cartridges.)
I then upgraded to the Koetsu Rosewood Signature. This resulted in more extended treble response and slightly better bass response too. Somehow the mid-range got even better. A truly great cartridge, with only one flaw, its somewhat loose bass response. Despite that this was a great cartridge, I really wanted better bass response, (as I had upgraded my speakers recently and I really wanted to take advantage of them).
So, I upgraded to a Shelter 90X. Wow! The bass response was incredible. The treble was well extended too. The mid-range was good, albeit not Koetsu good. The imaging and soundstaging was very good too. At first I was very happy with this cartridge as I thought it had everything I wanted in a cartridge. But it did not have the really low noise floor that both of the Koetsu cartridges had. After awhile I realized that I really missed those deep, dark, black backgrounds. So a friend who was using a Dynavector XV-1S offered me it at a good price.
The Dynavector has pretty much the best of all worlds. Great bass response, (nearly, if not as good as the 90X), and even more extended and refined treble response. The mid-range may not be Koetsu good, but it is close, and it meshes so well with the treble and bass response that I don't care! The imaging and soundstaging are incredible. And best of all, the noise floor was very, very low, resulting in those deep, dark, black backgrounds that I craved. I've heard several other top notch cartridges, but none really sounded much better, if at all, to my ears.
Good Luck in your search!