If you are looking for an alternate driver for the TL606 and would like to stick with the "classics" you could try a Gauss 4583A. They are available, but get one that's in working condition. They last a good long time, but original Gauss recone kits are very rare. Since the 4583 has dual spider, and four inch voice coil, I doubt power handling will be as great an issue either mechanically or electrically. Another option you have would be to run the TL606 in step down mode or retune the cabinet lower. This would roll off bass response fiercely, but would help mechanically. Remember the TL606 was designed for the EVM15L, not the 15B. The 15B has a slightly lower fs than the 15L, so a slightly lower tuning frequency would probably do the 15B some good without so much negative impact on bass. The gauss resonant freq. (31hz) is somewhat lower than the EVM 15B (I think), this might help a bit. Obviously there are more factors than fs in play, but usually (back in the day) when people were blowing up EV and JBL consistently, they replaced the drivers with Gauss. In my opinion, the electrovoice driver is a very nice sounding efficient machine, but the gauss has the advantage as far as power handling and durability, while keeping the efficiency. If you're reproducing bass, I think the gauss edges out the EV.
Over the years, I've had a whack of EVM 12L's and EVM 15L's both OEM and non-OEM. The "heatsink" is not really a factor in power handling, and I found it just took up volume in the cabinet. I usually removed the heatsink, and the little round plate that the heatsink held because the plate was a cause of buzzing when it got loose. In the case of the power handling, most of mine were 100 watts unless they've been reconed. Reconed, I was told they are 200watt. I assume this is due to the solution Electrovoice came up with regarding voice coil lead attachment mentioned earlier.
Cheers
Mike