You will likely not take out a loan of $40k. You're going to subtract the downpayment, subtract whatever trade-in value you got for your current piano, and use the rest as your principal amount to calculate your payment.
There are various very creative financing options out there that I have seen. A few examples:
1. Manufacturer financed 0% interest for one year. (You have to pay off everything in one year, or roll it over to some other debt facilities, such as those credit card balance transfer offers.)
2. Dealer financed 2% APR for first year, then 10% APR thereafter. (The trick is to pay off as much as you can in the first year, then roll over to something else.)
3. Straight financing -- that would be your 10% (+/- 2%) example. Still, after subtracting downpayment and current piano trade in, your actual loan amount shouldn't be that bad. With a bit of budgeting/spending discipline, paying the whole thing off in a few short years should be possible.
Lastly, think very carefully if indeed you can only be satisfied by spending $40k + trade. Not making judgment one way or the other, but after sampling a boat load of different pianos, may be there is a more economical yet unquestionably sufficient alternative that would present itself to you.