Small, Effective Subwoofers?

In a recent speaker thread that I started there were lots of suggestions of adding a couple of subwoofers to my system. As with my speakers (Legacy Signature SEs) I have little room to place subwoofers They will need to go directly to the left or directly to the right of the main speakers. To the left the maximum height would be 12". 12’ would be great on the outside as well, but I could squeeze a slightly bigger unit in there. Would two 12" subs be sufficient here, or would they be too small for this set up?

If 12’ is OK, do you all have any reasonably priced suggestions as did @nevada_matt withe his 2-10 offering from Tekton. I would have never found those on my own, thanks Matt!

I would be super open to some used subs as a cost savings measure here.