Threshold Amp and Sonic Frontiers Pre? HELP

I have a Sonic Frontiers SFL-1 with a Mullard tube and I am considering an amp upgrade to a Threshold S/200 Stasis. I wondering if any of you guys have expirience with SF and Threshold. I've been very happy with a tube pre SS amp combo thus far but I know that a Nelson Pass Amp will give a lot more to my speakers than my BK-ST140.

That being said, I love the BK-ST140. Maybe one of you will convince me to keep it in? I don't know. Any thoughts on that amp with the rest of my system?

I am using a pair of Theil cs1.5 speakers and a Monarchy Audio 18b Delux DAC connected to my computer. I'm using Audioquest Coral IC's and Audioquest Gibraltar speaker cables.

Thanks so much for any thoughts or ideas.