Subwoofer suggestions for budget system?

I think shipping is free? 

https://www.parts-express.com/8-Wireless-Subwoofer-Package-with-Dayton-Audio-SUB-800-300-5791

Any subwoofer is better than no subwoofer. 

[grislybutter,   'Is this true, always? Do I need a sub? If I mostly listen to music with little low frequencies (I think?) in a small room,']

Of course you don't need a sub...or two or eight. Many simply enjoy what the addition of subwoofer/s bring to their systems overall presentation regardless of the main speakers response. 

There are manufacturers that offer trial periods and the support to get the system dialed in.    

Capable subwoofer systems are a worthy investment. They're an audio component unto themselves that should complement any future speaker or system changes. The phrase, 'matching your subwoofer to your speakers,' can be a red flag. 

Hop off that fence. The sound down here is terrific.

That location seems questionable to me. Did you map your rooms modes and nulls using the crawl test with the sub playing tones at the listening position? 

Contacting SVS may provide some improvement.

Your Dynaudio"s are not at all the issue.

Where are that rooms bass modes?

With five corners in that room I’d be surprised if the mode you say your hearing between your speakers is a major culprit.

Unless you consider a subwoofer swarm no amount of equalization, room optimization processing or brand of subwoofer can solve the lack of placement flexibility and/or cable limitations you seem to be dealing with.

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.