I will play the devils advocate here. While adding a sub has the potential to add LF quantity and / or quality, it will invariably take away from the signal strength going to your main amp / speakers. That is, you need to make compromises at the low signal level, which might be quite steep especially when the subs plate amp with super low input impedance guzzles the current away from a tube power amplifier with high input impedance.
In easier to understand language: most of the energy that drives your system will be shunted to drive the subs, and your amp driving the main speakers will be starved and get only the meager leftovers that the sub graciously allows.
I have invariably found that adding a sub always has noticeably degraded midrange and high frequency quality in my systems, and the gains at the added bass level were not big enough to balance out the losses in the main frequency range.
I use very high efficiency speakers and straight path / no feedback system approach. However, when you have a preamp that has an added stage (or stages) to provide current drive for the sub(s) then this is a non-issue.
Yet, the problem of those extra stage(s) eroding low level signals still remain, so the main amp will receive compromised signals regardless a sub is connected or not. No free lunch, but we can plan when we understand the menu.