The rule of thumb as I understand it is no more than three devices, in your case amps, to the turn on lead without a relay. I'm running three Exile amps off of the HU lead no problem.
Another thing to do is run the HU power from the same source as the amps instead of the house wiring. David @ Earmark enlightened me to this saying that if you have the HU to house and the amps to stereo batt that if there's a differential in battery voltage it can damage the preamps in the HU. Easy enough to do, just draw power from the distribution block, in my case a fused Tsunami with an empty slot, or from a terminal strip connected from the stereo bank.