The 626 has cap coupled output's. Meaning that there is NO Protection Relay and there are TWO(2) 2A fuses for protection. The thump immediately upon power up is the coupling caps charging with DC leaking thru. The thump is the DC Voltage. This is typical for cap coupled amp's with marginal or failing caps. Look for the 2200uf/35V caps on the amplifier board(s). You can increase the value's of these to 3300uf/50V or 4700uf/50V.
Until you get the new caps, put it on a power strip. Then when you power down, turn the speaker switch to OFF, and turn off power at the strip. To power up, Switch on the power strip, count to 5 and turn the speaker switch to which ever speaker lead your's are connected to, ("A" or "B" or "C").
The manuals are @ hifiengine.com. You'll have to register, but it's free.
Mouser will have the parts.
Main Filter Cap.
UVR1J332MRD6 3300uf/63V QTY=1
http://www.mouser.com/ProductDetail...piMZZMtZ1n0r9vR22afkwEr%2bqyzvlw9tR0%2bo79c=
COUPLING CAPS
UPW1H222MHD 2200uf/50V 18mmx35.5mm QTY=2
http://www.mouser.com/ProductDetail...GAEpiMZZMtZ1n0r9vR22eD8a%2bFprAqc79oSbdIHPbo=
OR
UPW1V222MHD6 2200uf/35V 18mmx 25mm QTY=2
http://www.mouser.com/ProductDetail...GAEpiMZZMtZ1n0r9vR22eD8a%2bFprAqc8ccKvqogJ9E=
or UPW1V222MHD 2200uf/35V 16mmx31.5mm QTY=2
http://www.mouser.com/ProductDetail...GAEpiMZZMtZ1n0r9vR22eD8a%2bFprAqcC4CpvmWYBJo=
The last two are the same except for size.
for the Coupling caps on the amplifier board. I'd go with the 50V coupling caps.
This should either eliminate or greatly attenuate the thump. Again the thump is a byproduct of a cap coupled design. The speakers are connected to the amp all the time and when powered up, with marginal caps, you will get a thump, due to DC voltage getting by the cap to the speaker. When the cap is charged fully, the thump stops. That's why the recommendation of a power strip until you can recap the unit.
Larry