It's up to you to provide evidence for the fair value of your bag, but you will not receive a new bag as replacement for your old bag. The Montral Convention limits should no affect you, those are 1,288 SDRs for lost/damaged luggage and should cover most TUMI bags.
Take the 10% and then go to TUMI for an extra 25% off, everything else is for your travel insurance (if covered).