Audio research and krell

I believe krell added that jumper during the production of the ksa 250, because a certain model BAT preamp was causing DC issues, not all BAT models were a problem just one model if I remember right.

Yes that's correct I spoke to krell today the protection started on the fpb amps 

You can use your Ref 5SE into any Krell amplifier ever made.

Many years ago Krell said they would not warrant their amps when used with tube preamps because too many tube preamps did not have auto mute circuits (you had to have it manually muted on turn-on) and too many could output DC--which could blow up the amp. (Counterpoint, Melos, Audible Illusions, Paragon...) Even then, Krell would warrant their amps when used with ARC preamps because they were safe.

Your 5SE has coupling caps that will not pass DC, so you are fine.

Your 300S is over 25 years old and it is likely that some of the capacitors are showing their age. If the two halves are not discharging evenly when you turn it off then you will get some funny noises. If you turn off the 300S (with the 5SE muted) and you hear noise through the speakers, it's coming from the amp.

If you like the idea of older and big powerful amps, on US Audio Mart, there is a fella in the Santa Clara area (CA) selling a pair of Audio Research Ref 600 Mk IIIs in a black finish for $10,000 bucks I think.  That would probably mate up nicely to your ARC 5SE preamp.  When it comes to audio gear, if I can't move it myself, I don't buy it.  I seem to remember the Ref 600s weigh in around 170 or 180 pounds.  I bet those amps sound glorious though...