ORTF causes a loss of treble not bass. It's not really a loss, it's phase cancelation and it occuse at high frequencies where the wavelengths are close to the spread of the mics; bass wavelengths are much longer than the ORTF spread.
A tube pre with transformers should warm things up a bit but like muj said can also increase harmonics at the top end.
I'd suggest simply moving the mics closer to the sound source to increase the proximity effect and boost the mid-bass, this will in effect warm the recording.
One night, if you have enough cable, clamp booms on to each stack, split the mics - one per stack, tweak the aiming (degrees off-axis) durning soundcheck and give that a try, just keep at least 2-3 ft off of the stack to ensure a mixing of the horn with the mids.
peace, chris