You need to buffer the input with another pedal before the delay. If you want to continue using a "stomp-box" type delay, I would try the Boss RE-20. It sounds fantastic, and will internally buffer the signal via its input level select. You can also experiment by putting a 5751 tube before the effects loop to "cool-off" the input signal. If I'm not mistaken, MESA is loading the MKV's with JJ's 12AX7 tubes? These are "hot" tubes to be sure! Remove all the 12AX7 tubes, and replace them with Sovtek LPS 12AX7's. These are long-plate preamp tubes, with a slightly lower output. They're also a more musical tubes than its JJ's short-plate counterpart. You can swap the V1 valve with a JJ's ECC83, a Sovtek LPS. or a Jan Phillips 5751. Don't limit your options I always say mate! I use a Jan 5751 (SRV's secret weapon) in the V1 of my MK IIC+ exclusively. I do carry a JJ's ECC83 (12AX7) in the spare tube bracket - just in case. 8)
This reminds me... Will everyone please call MESA, and suggest:
1. Repositioning the V1 socket (in the rev B models) so tube swaps aren't so GD difficult!
2. Give channel 3 a "pull/deep" option. I can't believe they forgot to put that on channel 3! WTF?