For any FirstWatt just be sure you aren't sharing ground. You have to wire L+ L-, R+ R-.
As for the LF v Peak, I never had both at the same time and originally based on reviews, had no intention of trying a Peak. That said the LF with 'good' tubes is $2400+ used, $3400+ new. At that price merely hanging with the best isn't enough, you need to slaughter the competition. When I started my trip through amps for the LCD-2 then LCD-3 I already had the GS-1. Brought in the WA22 and LF and compared them against each other before comparing to the GS-1. The WA22 had arguably better tubes but the LF was just too passive, too dry and too fuzzy to keep around. Based on what I heard, it really belonged in the sub $1000 range. The WA22 was simply more alive, more detailed and reached further in both directions.
Later the B22 would top the WA22 as a similar but with better soundstage offering that in my config also had pre-amp so no brainer new best choice. The Peak simply out classed them all. Where the others were incremental improvements, the Peak was simply better everywhere possibly due to tube choice or maybe a change was made to the original, I don't know, I don't care. Since I've also tried the Soloist which is like a better GS-1 and the Mjolnir which is like a Peak that's been tonally tilted up.
If I had to make a choice on an amp under $1000, the Mjolnir and Soloist are no brainers, Soloist for a more passive presentaion, Mjolnir for an engaging presentation.
Above that the stand out is the Peak for me. The other amps I tried above the $1000 mark were bested by the Soloist and Mjolnir. I haven't heard a lot of other top choices though so an EC BA, Woo WA5, GS-X, Dynahi... may well crush the Peak. I may never know, I've moved on to source swapping :D