Firstly, The Fender 300T is an exceptionally rad amp. A lot of people will tell you its to much power yada yada yada. I have had people tell me not to buy things because of there opinion about how loud it was or how i didn't need to spend that much also and than later in life spent more money cause i bought something they said would work. Sometimes you just need to buy what you want once and if its over kill at least your prepared for any situation. The 300T may not be originally Sunn, Im not completely sure about that, but it is popular in that style and almost every other style of music. As far as cabs go your just going to have to try them out. I own the matching 8x10 to that head and love it. An Ampeg or something along those lines would be nice also. Honestly what i would do is try that head out on a 8x10, a 2x15, and maybe even a 6x10. Decide what tones you like. Noted, not all cabs sound similar just because they have the same speaker set up. But they have similar qualities.
Rest assured Johnny, it's been a FMIC product from it's very inception. As I posted earlier; "that badge is the only thing "Sunn" about the Sunn 300T".
Not that that's a bad thing per se; as I also mentioned IMO it's a pretty nice amp.
Also, food for thought, if you like your bassman now you really should check out the new fender line of bass amps. Fender make some really really nice stuff. I would be willing to put most pro line fender amps against other people boutique crap. People just forget fender is trying to sell to many groups of musician and there beginner stuff is horrible. And there mid of the line stuff is sub par at best. Mmmmm, that pro stuff really sings to you though.
Well, you could put most pro line fender amps against other people boutique crap, but it wouldnt be a fair fight. Quality wise its not a contest; that mass produced PCB amp isnt even in the same league with a hand wired boutique amp from a craftsmanship perspective, relative to market value, or from an ease of service/longevity outlook.
As far as tonally; well, of course there is just no accounting for taste.