I’ve been using a sub for the past 15 years or so and never really got a satisfactory result blending it to the main system.
What sub? What controls did it have?
I've heard this idea that only Rel subs can be easily "integrated" into a 2 channel system mentioned a lot. Frankly, I don't get it.
Most modern subs include a lot of options for setting phase, crossover, Parametric EQ, and more. Integrating a sub (or multiple subs) isn't rocket surgery. Many now come with remote control apps so that you can adjust parameters in real time from your listening position.