It looks like you have made up your mind here on sticking with the LX-380. I came across your forum in considering luxman tubes with falcon kit ls3/5a variation, 15ohm impedance. For what its worth, I have been using my L550axII (20 watt rated, but measured in one hifi magazine as clipping at near 70watts) pure class A. Based on my use of the 550 with three speakers (Original Cornwalls, ProAc Response D2, and these falcons), the description offered above (They can drive speakers with authority. The overall signature is balanced with articulate midrange and treble. Bass frequencies are tight and resolving) is dead on.
With the original cornwalls, which are quite warm on balance with rolled off top end, the 550 is excellent and could blow the windows out with volume. With the Proac Response D2, which I find to be warm on balance but with a slightly hot top end, the combo sounds wonderful and refined, but somewhat warm and dry. Good recordings sound great, bad recordings sound bad. With the Falcons which are bright on balance (too much treble energy IMO, but exciting), the 550 accentuates dryness and graininess in bad recordings, which is most of them. It could use some taming/smoothing out.
So I am curious how you ended up liking the 380 with the ls3/5a?