genek

Sold for 600 Euros.  Wonder what shipping to the US would have been.

The Euro AR-6 with three-position switch uses a single 24uF 30V capacitor (the model with the 10uF cap only has a two-position switch).

You may or may not notice much change in the sound if you change the caps now, but 10 years from now is another matter.

Minimum rated power for this model is 20wpc, a bit less won't be a problem in a small room and not very high volumes.  Maximum rating is 100wpc.

Inadequate sealing normally produces reduced bass.  Breakup sound at moderate volumes indicates an issue with the woofer.  I would start with that less than stellar refoam.  Whoever did it may not have properly centered the cone. 

Pictures definitely needed before advice about cabinets.

I think the simplest approach to the gaping hole would be to cut a round piece of masonite, paint it matte black, drill it for the lugs and level controls and glue it over the existing board from the outside.  The three wire lugs would be easy enough to loosen and retighten through the new board, and there should be enough length on the threaded posts to accomodate a 1/8" thick additional face.  The white lettering was simple enough, and you can find rub-on lettering that you can use to recreate it.  Or, you could leave the lugs alone and just make a partial circle to cover the lower third of the port.  It's not as if this is going to be seen a lot.

However, if you're determined to do the whole replacement, it doesn't have to be a board from an AR5.  Any AR 3, 3a, or 2ax crossover board will do as well.

By the mid seventies, even "boiled linseed oil" was no longer just oil, but a mixture of oil and various solvents and spirits designed catalyze the mix to speed drying.  Not a commercial varnish finish with resins added, but the catalytic action produces a hardened polymer that chemists do refer to as a varnish. 

If you compare an AR speaker from the 70's with one from the 60's, the 70's finish definitely has a slightly harder feel and more of a "sheen."

Watco Danish Oil is an oil-varnish blend, which contains oil, catalytic spirits and varnish resins.

AR did not use varnish.

Kent

Touch up pens.  They're basically indelible markers in wood shades, but material doesn't need to absorb them.

Next time you need to do this, look into burn in sticks.  You blend them for color and use them to fill gouges and deep scratches after the wood has been finished.  Wood filler never absorbs stain the same as wood.