Would it be okay to plug the Rel Storm 3 sub bass unit and the preamplifier into that dedicated line along with the two monoblocks, or would that screw up the isolation?Yes, it would be okay since these are all analog components (your preamp is analog, and there's no digital amp in the Rel, right?). Also, you might consider purchasing a Hydra 2 (remember to ask for the supplied Shunyata power cord), and plug your Rel and preamp into the Hydra 2, which is plugged into one of the four outlets in your dedicated circuit. This would leave one outlet on the dedicated circuit into which you could plug your Original Hydra. At least this way, you could play around. One other idea is to purchase a used VH Audio HotBox 4 Outlet extension box to use in lieu of the Hydra 2. This could be used for your Rel and preamp, too. I saw one for sale recently on Audiogon.
Couldn't a second dedicated line be run next to the one existing dedicated line? After all, you already have a four outlet gang box in the wall. Plugging all that equipment into the same line as your amps seems like too much to me, although I'm sure the line has sufficient amperage to avoid tripping a breaker. Still, it's nice to have extra juice for those dynamic transients.