Subwoofer suggestions for budget system?

REL -6dB sub bass speakers are limited to de-tented crossover and phase control. Reducing volume is their most effective control. 

The three bare wire high-speaker level to the amplifier output connectivity  (yet another input/output circuit for the source signal to pass through, limited by the amplifiers low frequency response)  was originally used, and is a convenience for customers lacking preamplifier outputs as do many other subs. Balanced output be informed.

You'll find Mr. Lord's unchanged connectivity now produced offshore is described as system and room integration lauded by mostly new owners without any other subwoofer experience.

Even the practically foolproof and more economical Swarm system has one band of parametric equalization.

On the other hand: if your speakers are very small or your amplifier is under powered, you may find the speaker level connectivity more desirable. You'll still need to deal with any room frequency issues. Those chrome bits and shinny finish are attractive.  

that's the last step indeed, using the speaker connections. I am a bit reluctant since the sub has a monster amp but still, there would not be any phasing issues. 

@grislybutter Have you tried switching the phase on the sub to see if that yields any improvement?

@soix I did, I took a little break, I am back to tweaking again. I just figured out that  phasing and placement go hand in hand (and then I read up on it and confirmed it) baby steps :)