My experience adding subwoofers to 2 channel

I recently purchased some transmission line (like) ported towers and was unable to successfully integrate my single sub- so I sold it.  
I have had no problems in the past with typical ported monitor speakers- and am quite good at integration using RTA, phase control for time delay, etc.  
I could not get the bass to sound good- often hard sounding despite my efforts.

I had no peaks in the response either so EQ will not help.  

I'm guessing that transmission line ports are somehow more difficult to integrate.
I miss the impact my sub provided and any advise would be appreciated.  

Tim,

Thanks.  I'm going to start a thread on this topic.  

My experiences tuning subwoofers with main speakers is that the phase timing is critically important at crossover blend region where the subwoofers and main speakers transition between each other.
An 80Hz wavelength is ~ 14 feet long and is small enough that room dimensions, distance between subwoofer and main speakers as well as distance to listening position can affect the quality of bass response.
Even with a 40Hz crossover frequency there is still significant energy at critical mid bass frequencies (e.g. 80-120Hz) being broadcast by the subwoofers and main speakers. If the timing of the subwoofers is not adjusted accordingly so that the mid bass frequencies aren’t optimized then the listener will hear weak midbass from phase / timing cancellations.  The deep bass will be OK but there will be a gap in the response such that the impactful midbass frequencies are relatively weak.  
The only way I have found to get this right is to use tracks with powerful bass beats and adjust the phase of each subwoofer until it sounds the best- (by ear). With experience you’ll know it when you hear it.  The impact frequencies in the midbass will sound effortless even at low volume levels.  The key is in the midbass.     
DSP, room correction etc. fall short of achieving this automatically.