You might consider putting some conventional fluorescent tubes in it. Depending on the size of the softbox, various layouts might work. Perhaps these would suit: they're only two feet long, rated at 18W each. It's possible that the low-cost CFLs you've been using don't really consume 75W so you may find five or six enough to replace them. You'd need to budget for holders and ballasts, but the wiring is reasonably straightforward and you can swap the tubes out to change colour temperature. I would recommend budgeting also for shatter sleeves for them, as are used in the food service industry. They don't make the tubes much tougher, but they do contain the mess.
The other way to go, if you're feeling only slightly more ambitious, is 150W ceramic metal halide. They come in single-ended G12 or double-ended RX7 types; 5200K is more common in RX7. This is the technology used in retail premises and sold by Cool Lights in their 150W CDM fresnel. They have broadly the performance of HMI and you can get them in various colour temperatures, and party colours. The only downside is that they don't hot restrike on conventional ballasts. Look out for colour rendering, though; they're available in every grade from desperately cheap to very expensive, and performance changes accordingly. Again, ballasts are required, and there is a brief warmup time. They should not, in my view, be used outside of an enclosure sufficient to contain fragments of glass should a lamp fail.
There are larger metal halide lamps available in 400W and above which might suit, but I'm not experienced enough with their colour performance to recommend them.
Oh - edit - if you go for metal halide, get the electronic ballasts and they're admirably flicker free.