Russ, Mr. Dodd passed away recently. Give his good friend Danny Richie a call at GR Research.
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Danny Richie and Gary Dodd go way back together, and were actually in the planning stages of a new company together. Danny knows all about the Dodd products, having both a Dodd pre and a pair of big mono amps Gary custom made for him. If you ever need any repairs made to your Dodd pre, Danny can take care of that for you. Plus, he's a really down-to-Earth, nice guy. He's also a speaker/sub designer, so he can advise you on your sub hook-up.
10-06-15: 1markrAlthough that is nearly always the case :-) it may not be in this particular situation, for the following reasons which might in fact account for why you prefer to run your ML sub from the amp.
More often than not a preamp providing two sets of outputs drives both of them from the same output stage, with the two pairs of output jacks wired directly together just inside the rear panel. Also, many and probably most tube preamps have very substantial rises in output impedance at deep bass frequencies, commonly to 3K or 4K or more. Also, the input impedance of the line level inputs of many powered subs is low, often somewhere between 5K and 20K.
The combination of all of those factors, plus the fact that the same preamp output stage is likely to have to drive both the sub and the main power amp, can be expected to often result in a significant rolloff of the deep bass content of the signals provided to both the sub and the main power amp.
Russ, if you contact Danny (good suggestion by Bdp24!) see if he can tell you what the output impedance of the preamp is at 20 Hz. And it would also be helpful if you can determine the input impedance of the line-level inputs of the sub, and also if you can confirm that it can accept two input channels, rather than just a mono input.
Regarding speaker-level connections, assuming the sub can accept two input channels the one possible concern would be if your power amp has balanced or bridged outputs, or if it consists of two monoblocks. In those cases there are special measures which should be taken regarding connection of the negative/black input terminal(s) on the sub.