Hi, I have a 3 channel Mesa Dual Recto which runs 4x6L6 and 2x5U4 as stock. I prefer to run the amp with 2x6L6 (observing push-pull operation) and 1x5U4. I use the 4 ohm tap with an 8 ohm speaker. I always use the 'spongy' AC power setting. All together, it sounds pretty cool. Would it be safe to run 2x6L6 and 1x5Y3? Why exactly does a 5Y3 'sag' more than a 5U4? Thanks!