Below is the web address for the FPB-700cx:
http://www.krellonline.com/archive_pdfs/FPB_STEREO/0162_020_MAN.pdf
Here are some relevant excerpts:
POSSIBLE CAUSES
Excessive DC at the
amplifier input
Electrical short in speaker
cables or inside speaker
Insufficient AC current from
the wall outlet
Excessive heat
DC present at the output
stage
Internal amplifier problem
COURSE OF ACTION
If preamplifier is a tube
model, see Using a Tube
Preamplifier, on page 14.
Check all speaker cables for
any cuts or frayed edges that
might form a connection
between the positive and negative
speaker binding post terminals
Make sure the AC line is at
least 15 amps for 300cx and
350Mc amplifiers; 400cx,
450Mcx, 700cx, and 750Mcx
amplifiers should be
operated from a dedicated
20-amp line.
Allow amplifier to cool before
resuming operation. See
Placement, on page 4 for ventilation
requirements
USING A TUBE PREAMPLIFIER
The high DC output of tube preamplifiers may
exceed the DC protection circuitry of Full
Power Balanced amplifiers. Excessive DC
level in a signal can damage amplifiers, speakers,
or both. The coupling capacitors in Full
Power Balanced amplifiers must be engaged
when using a tube preamplifier. An authorized
Krell dealer, distributor, or Krell must activate
these capacitors.
Coupling capacitors must be inserted into the
signal path by your authorized Krell dealer, distributor,
or Krell before you can use your Full
Power Balanced amplifier with a tube preamplifier