I almost feel like the best option for what you're looking for might be the new Ridiculous Asym or a Guru Mighty.

I think the Braniac would be too angular and the Einstein might be a little too strong.

Or drill a Melee Jab and change the surface to 3000 no polish

I'd say the Melee Cross would be a ball that fits inbetween.
I'd also agree with the previous poster that the Melee Hook would be a great option too. It would be stronger than the Cross and would probably be the better option versatility wise. 

I would agree with the melee hook or a melee jab if you want a little more snap downlane.

If your Nirvana is your big ball, I have to imagine you have a pretty large step down before you get to the LT, which means that there are several balls that could fit between the two. If you're looking for something smooth, the Melee line is really good there.

I didn't get along with the Cross as well I have the original and the Jab. The Jab corners pretty hard, so if you want something smoother, I'd go with a Hook, or I'd even consider taking an original down to 1000 or so matte. It makes that ball an awesome benchmark piece.

You could also consider a Soul with some surface. The Soul is another solid symmetrical piece, and those usually lend themselves to being smoother. FYI, I've liked my Soul with the box finish knocked off to make it a touch earlier.

If you want to go outside of the actual Brunswick name, the Ridiculous has looked phenomenal in the hands of just about everyone I've seen throw it. It it were me, I would consider knocking some of the shine off in order to smooth it out a pinch, but I think you'd be okay either way.

Long story short, depending on your style and desired motion, I think that there are lots of good balls in the range you're looking for.