rca single from pre-out to y at sub in---no problems and sound quality and speed great but if you're running 15 feet or more to your sub you should consider balanced xlr
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Depends on the preamp. XLR output is pointless, as DD has only RCA inputs (except for one XLR LFE input for HT.) A Y-connector bridging L/R outputs of preamp works only if the preamp outputs are internally buffered. Otherwise outputs to main amp will also be bridged into mono.
Because my Atma-Sphere preamp briefly emits DC off-set when it's turned on, I buffer line-level outputs to DD sub with Jensen Isomax coupling transformers optimized for LF use. But this is not a typical configuration.
As I'm in the process of putting my rig back to gether again just this week,I've coupled two sets of no name RCA ICs using only one side from my Onkyo Receiver...
In the past I've run via this same cable (s) from both my Bel Canto DAC3 and Thor preamp. On the Thor I had to use a pair of "Y's", as it has only one set of active RCA outputs.
I've detected no adverse issues in the sub's sonics in any of the above configs.
I will be spending more time this go 'round in placement, and will ultimately go with one run of RCA cables to the sub... personally, I like the idea of using a preamps outputs, rather than an added set of speaker cables.
As I mentioned, the DD has one XLR input for mono LFE. With a 2CH preamp that has a pair of auxillary XLR outputs that are internally buffered from main outputs, an XLR Y-connector could be used between the preamp's auxillary outputs to bridge L & R channels for XLR input to sub. However, this will not work with the many 2CH preamps that wire auxillary outputs in parallel with main outputs. In those cases you must run both channels across via RCA inputs to take advantage of the sub's ability to combine them into mono while buffering L from R with respect to the preamp.
A surround-sound pre/pro/receiver is a very different animal, insofar as the LFE output internally bridges and buffers L & R, applies a fixed crossover frequency & rolloff, and may EQ the sub signal in accordance with THX, etc. Since using LFE implies that a fixed crossover point has already been applied to the signal upstream by pre/pro/receiver, I'm curious whether using XLR input into DD disengages the ability to manipulate crossover point & rolloff within the DD's systems menu. This would be undesireable in a 2CH application.