I think Chris really wants to have the instrument tuned to bass tuning. What you (Chris) have done is going to be hard to improve upon without some real science and luthiery. Your string length is only 65% of what those strings were designed for, so the tension is proportionally lower. (pitch is a function of string mass, tension, and length) You might try a heavy gauge (stark) string on the bottom to increase tension. A more flexible string, like a Helicore orchestra (stark) might work better under the reduced tension (It works on my student's 1/4 size bass anyway). Another way to go would be to tune up by some interval, but the players might find this too confusing. A third option would be to reset the neck at a steeper angle and install a higher bridge. I doubt the bar will finance such a venture, so that's out. You do want to think about the overall tension on the instrument. I don't think what you're doing will be a problem. You probably have less tension that the cello was designed for, but with some research, you could figure out what is typical for a normally tuned cello, and what the result will be for your string choices. To do this, you would have to figure out the mass per unit length of the various strings. Short of all that, try to get a Helicore stark "E". That might solve all your problems.
Good luck!
Robobass