I understand what the local tech is talking about but IMO he should be at least willing to hook up one or both subs without the crossover to see if it can be blended properly without introducing a crossover into your very refined analog electronics.
First, I suggest if you have not done so, download your sub OM and read the section starting on p24. This what the local tech is referring to:
"When connecting a Fathom (or multiple Fathoms) in mono to a two-channel audio
system you will use both the “Left or Mono” and the “Right” inputs. Summing circuitry
in the Fathom’s input section will sum the stereo signals to mono. We strongly recommend that you use a high-quality active crossover to divide your preamplifier’s signals prior to connection
to the Fathom and to the amplifier driving your main speakers. This will allow you to filter low frequencies out of the signals driving the main speakers, resulting in better performance."
(Ed: I will add that a crossover consists of a combination of 2 filters. The low-pass filter splits off the freqs below a given point-say 80 Hz-and send those to the subs. The high-pass filter sends all freqs higher than 80 to the main speakers.)
And the very next part is what the dealer is referring to:
"If you are not using an active crossover and are comfortable running your main
speakers full-range, you can split your preamplifier’s output signals using appropriate Y-connectors in place of the active crossover shown in the diagram. "
But let's step back a moment. First off you are completely right, what's important is really whether YOU'RE happy with the bass. And ideally you'd only want the equipment needed to get you there. If you hadn't a faulty driver, would you even be considering subs? I think not.
As I'm sure you recall, early on in this conversation I pointed out that your processor does not have bass management (a built-in crossover) and that is not ideal if
you need subs to fill your space with nice bass. If you really don't need subs, you have a different problem-returning the subs for a refund.
Ideally the Fathom dealer would accept the subs back and the Epicon dealer/SoundOrg would cover any restocking/shipping fees to make up for their sheer incompetence in diagnosing an obvious problem. Less than ideally, perhaps the Fathom dealer will take one sub back as it's sounding increasingly unlikely given the circumstances that you'll need two.
Given your preamp I early on suggested basically the same solution as the Fathom dealer is, i.e, using the sophisticated electronics in the sub(s) to blend the sub(s) with the mains simply by using the low-pass filter in the sub. Yes the mains have no high-pass filter and will be putting out some bass as they are receiving the full spectrum signal. That could make for boominess in the range of 60-90 Hz. However, the built-in EQ of the subs should compensate for that.