play a sine wave up through the low frequencies, if you are feeling cunning you can figure out what frequency you are listening to and then compare that against your room mode calculator. should line up more or less, also you can hear any big problem freqencies and make allowances when you are mixing your bass in that key.

stuff in A sounds horrible in here.

and you can always just go with the sine wave solution and just move the speakers around till it sounds best.

there's an mp3 sine wave rise at www.genelec.com you can use if you haven't got a synth that gives a good linar slide up..