sub for KEF q100 bookshelves?

Hello - first time post, I'm a 'newbie' I guess and I'm on a student budget.

I bought a pair of KEF q100 speakers and love them, after auditioning as many things as I could at sterio shops at home over the holidays. For my price range and musical taste, these sounded by far the best, so I bought them ~$500.

Now I need to add a sub, and where I live now (DC area, which I am new to) I can't seem to find a sterio shop with subs in my more 'modest' price range to audition. Suggestions on stores?

What would you all recommend in the entry-level range for best bang-for-buck sub? Would the KEF KUBE 1 or KUBE 2 be good for music listening?

I have two amp possibilities, a Onkyo A-9711 with both speaker and line level out, and an old PSE Studio IV I found with only one speaker-level output.

a specific newbie-question: IF I got a sub that only took line-level input, for example, and later I bought a preamp so I could use the PSE, would I be able to connect one preamp output to the sub, and the other to the PSE Amp?

Anyone know where I could get the matching Studio SL Preamp, or have recommendations for something else?

I have a Entech 203.2 DAC and a Headroom Total BitHead DAC/headphone amp, as most of my music is on my laptop. The Entech sounds much better to my ears.

thanks for your help, any/all suggestions welcome, go kindly though as I'm new to this.