XLR or RCA for my new DAC

XLR!!

I use XLR as my preamp is balanced and I get more gain. If single ended, I would use RCA.

It only matters if the Xlr are truly balanced from input to output 
on the dac as well as the preamp , I have been  involved with audio over 40 years ,on most ,if you pop the cover the rca, and xlr are connected together before the output  in the chain on over 90% of all units out there , it’s a nice selling point showing both rca,and xlr.
for if it’s not true balanced that extra ground on the xlr just connects  into the 2 wire outputs ,True Balanced will have transformers for inputs and outputs which is very rare .I have had balanced  cables and the rca same brand had better imaging, the xlr had a bit bigger soundstage a hair quieter depending on the design. It’s not a straight forward conclusion.

OP, happy for you to have found the magic a good DAC can bring. I love it when people really get a noticable improvement on a piece of gear. That bifrost has multiple inputs I believe. Could be a good opportunity to explore the world of streaming. Happy listening and keep posting your impressions with the DAC, they are fun to read. 

Kyle 

That Bifrost 2 is a good starter DAC. I have one in my collection. Look there are a number of things to consider regarding SE and balanced connections and cables. This was noted above. However, all things being equal I would go with the XLR connections and you are paying for those XLR connections. XLR connections are new to the Bifrost with the Bifrost 2, so you might as well take advantage of them.