You are asking many different questions.
"anyone knows if i can use McIntosh power amp/preamp/integrated bought in US (110vac) in Japan - 100 vac" I think you should ask McIntosh this.
"can McIntosh convert or it has some jumper/swich inside to change to 100 vac" I had my MC275 serviced at my US Mc dealer who opened it up and changed some jumpers to make it a 220V unit, so this might be the case with most/all McIntosh units. By the way, McIntosh was telling me this could not be done!
" is McIntosh will still service US unit in Japan?" Again, you should ask McIntosh, and/or the Japanese authorized dealer. I would think so.
Then about speakers and amps, do some search here in the forums. For B&W 801 you will want a large solid state amp, probably a McIntosh MC402 as a minimum. McIntosh & B&W are generally liked together.
"anyone knows if i can use McIntosh power amp/preamp/integrated bought in US (110vac) in Japan - 100 vac" I think you should ask McIntosh this.
"can McIntosh convert or it has some jumper/swich inside to change to 100 vac" I had my MC275 serviced at my US Mc dealer who opened it up and changed some jumpers to make it a 220V unit, so this might be the case with most/all McIntosh units. By the way, McIntosh was telling me this could not be done!
" is McIntosh will still service US unit in Japan?" Again, you should ask McIntosh, and/or the Japanese authorized dealer. I would think so.
Then about speakers and amps, do some search here in the forums. For B&W 801 you will want a large solid state amp, probably a McIntosh MC402 as a minimum. McIntosh & B&W are generally liked together.