No you can hook up some subs directly from the speaker outputs on your amp. I believe REL subs are designed to be used this way.
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You can use a sub that has a speaker level hook up like the REL that is designed to sound best that way. You would run them in parallel without any degradation. The REL will actually emulate the sound of your amp to make it seamless.
You can use Monster solid brass 1 in 2 out splitter and as long as you use a left and right one to drive the sub in stereo, your output on your digital source should be able to drive that load with no issue.
Both of these options are better sounding than most outboard crossovers.
Sealed speakers with a second order sub-woofer high pass will mate well. Anechoically you'd want the roll-off points to match, although in your room you may want asymetric roll-offs.
If you have enough adjustability in the sub-woofer low pass filter, you could just split the DAC output with one connection to the power amp and one to the sub-woofer input. If not you could use a passive line level solution, sold commercially as "fmods" for car-audio.
Ported speakers unload below the tuning frequency and really benefit from electronic removal of lower frequencies.
You could probably use the same splitter with a passive line level high-pass on the power amp input. Many sub-woofer amplifier ubnits include high-pass filters, but they tend to not be very adjustable.
An external sub-woofer cross-over would be ideal.
Assuming the output section of your DAC is up to it (i.e., driving low impedance loads) simply split the output between the amp and sub.
Or, depending on your main speakers (if they're stand mount), run the DAC's output to a bass management controller and then to the amp and sub.
In either case, a preamp is not necessary.