Limited fine-tuning of planet/star properties

[jswear]

I'm attempting to turn a "Random Gas Giant" into an 8-Jupiter-masses brown dwarf, similar to: http://en.wikipedia.org/wiki/Cha_110913-773444

If I punch in "8(J)" for mass and lock that value, and then "1.8(J)" for radius (like seen on the above Wikipedia page), the radius will immediately jump to ~2.5 instead of adjusting the density accordingly. The density lands on a value of ~690 kg/m3, which won't adjust negatively or positively.

The temperature also appears to be completely fixed (although I know you guys are already looking into this one).

Conversely, if I start with a red dwarf such as Wolf 359 and give it a mass of 8(J), the radius jumps to 1.79(J), which is perfect as far as I'm concerned.

So I guess the real issue is that while I can modify a star with ~95 Jupiter masses into a finely tuned brown dwarf, I can't do the same with a gas giant that has a much closer starting mass.