I have a Furman RV-1 spring reverb for sale. It has the red knobs and limiter. In good shape for it's age. Line level or instrument level. I use this after the M4 and before the RT2/50. It also has a jack for a footswitch to turn it on or off. Great sounding reverb. $225 paypaled and shipped.
Pics here: http://www.rig-talk.com/forum/viewtopic.php?f=6&t=55818
Pics here: http://www.rig-talk.com/forum/viewtopic.php?f=6&t=55818