It seems like you're overall happy with the sound you're getting but just looking to augment the bottom end a bit. That seems to be common with your speakers and what the SC4a may have addressed. I doubt spending $4k on speakers, amp, or pre is going to get you what you seek without potentially compromising other areas. If I'm you, I'd demo a sub with a good bass management system and see if that adds what you're looking for. If that helps I'd then get a second sub and live happily ever after -- although I'd eventually replace the preamp as well. Best of luck.
Upgrade path
I'm currently running a set of Dunlavy SC4 speakers bi amped into a Theata Dreadnaught 1 amp with the 5x200 watt set up, Preamp is a Audio Research ls3. Looking for opinions on what I should up grade 1st of the 3 pieces. I was thinking a bigger amp in the 400 watt range but one that would be an noticeable improvement over the Theat which to my ears does a lot of things right. Amp wise I would not be looking to spend more than 4K