Well, I can see that you are not getting any input, so I will give you what input I can, based upon my experience.
First, a bit of history on myself. I too love the Koetsu sound. It is sort of like listening to tubes versus solid state. There is just something there that brings out the soul in the music, as it gives it an organic quality that some cartridges just seem to lack. Of course it has its weaknesses as well, especially in the bass response, which is rather loose, and not as well extended as others. (I have not heard the stone bodied Koetsu cartridges, but my friend had a Rosewood Signature Platinum, which while better than the regular Rosewood Signature, was still light in the bass response, IMHO.)
In addition, I also like the Basis turntables, as they are very well built, and have a very well thought out suspension system. (I have a little girl who loves to dance to my music, so it helps to have a great suspension, which eliminates the effects of her footfalls, even though it is within a couple of feet of the turntable!) I started with a Basis 1400, w/RB300 tone arm, and have since moved on up to the 2001, with a Vector M3 tone arm. At some point, I hope to upgrade to the Basis 2500, but I need to save up some money first.
My first cartridge, upon getting back into vinyl about 6 years ago, was a Benz Micro Glider II, which was an acceptable cartridge, but nothing to write home about. After a cantilever accident, I replaced it with a Koetsu Black, which really opened my eyes as to what vinyl was all about. I was hooked! The incredibly black background was intoxicating. A year later, I replaced that with a Koetsu Rosewood Signature, which tightened up the bass, a bit, and really extended the treble response. I really liked it a lot, but I found I was still missing the deep bass response that I wanted. So after another year or so, I replaced the Rosewood Signature with a Shelter 90X. (The Shelter was a good cartridge, especially in the bass response, where it was great actually, but its mid-range, while warmish, just did not have that lush Koetsu quality. In addition, it did not have the deep, dark, black background that the Koetsu cartridges had.) After awhile, I longed for that deep dark black background and the lushness of the Koetsu cartridges, so I started saving up for the legendary Koetsu Jade Platinum, as I had discussed its attributes with various other audiophiles here on Audiogon, and figured it was going to be the cartridge for me. (However, it is very expensive, even used, so it was going to take me awhile to save up for it.) In the meantime, a friend of mine picked up a Brinkmann Balance, along with an EMT cartridge. So, he sold his turntable, and offered to sell me his cartridge, the Dynavector XV-1S, at a very good price. I figured "why not?", it sounded good on his VPI HR-X, so I bought it, figuring that it would tide me over until I could afford the Jade Platinum.
Well, after I installed the XV-1S, I was so enthralled by its sound, that I have now forgotten about upgrading to the Jade. (Not that it does not intrigue me; it is just that I am so happy with the XV-1S, that I have no desire to move on.) The bass response is great, (nearly as great as the Shelter 90X); the treble response is well extended and highly refined, (much more refined than the Shelter). And the mid-range, while not Koetsu lush, has an organic sense to it, and it just seems to blend so well with the treble and bass response, as to make a seamless tonality that is just wonderful. The soundstaging and imaging are incredible as well. And, best of all, it too gives me that deep, dark, black background that I loved about the Koetsu line. The Dynavector XV-1S may very well be the last cartridge I ever own, it is that good. So my guess is that you too will love it. (FYI: I have no experience with the mono version, but if it is anything like the stereo version, I imagine that it too is fantastic.)
If you really want to stick with the Koetsu (and who can fault you? - I certainly won't!), then as far as your turntable goes, are you aware that you can mount another tone arm on your 2500? (Basis uses a 2502 designation for such a model.)
Basis 2502 I "assume" you could retroactively modify your existing 2500, but you can contact A.J. to find that out.
As far as tone arms, I can highly recommend the Basis Vector. It is not as flexible as the Phantom is reputed to be, but sonically, it is top notch, and it is very simple to use and set up.
Those are my two cents worth.
Feel free to contact me if you wish to discuss this further.
Good Luck!
