@DB-7, don't let this discourage you from buying from Japan. Granted, I have had limited experience (5 total transactions so far) buying from sellers located in Japan, but all have gone smoothly and I received the gear I purchased in good shape. You could get screwed just as badly by a seller in the US or Canada with poor shipping and an item being misrepresented as regards it's actual condition.
The major downsides to buying from Japan are shipping costs (though sometimes that is still free) and at times the communication is a bit challenged due to the language barrier. Japanese sellers I've dealt with this far have had acceptable written English, and therefore misunderstandings were thankfully avoided. But I can see where that isn't always the case.
Getting back to this mess of an amp, is the glue like materials semi-soft and flexible, are is it rock hard? If the latter, I almost think it looks like construction adhesive (Liquid Nails being the common well known trade name in the US). If that's what it is, removing it will be possible only through mechanical means (scraping, cutting, sanding) as the stuff is mostly impervious to solvents once cured. Acetone may take off small, thin layers of it, bit that is about it, to my knowledge.
If it is flexible, it could be silicone/RTV type sealant (plumbers caulk, or automotive "gasket maker"). This too is mostly unaffected by solvents, though once the heavy portions are removed, denatured alcohol will aid somewhat in rubbing the rest off fairly cleanly.
