have you ever bought stereo without listening first

Never heard but they have a sterling reputation. You have to look at the design features (usually stated in reviews or maker’s website). The big elements with amps are: Class of operation e.g. Class-A, class-AB, class-d, etc.. Each class has positives and negatives. Next is global feedback. Global feedback, when used in excess, will make the amp’s specifications look great but can make it’s sound "uptight" or closed-in, constricted. My amp uses no global feedback. Finally, you must know if the amp will "work and play well" with your speakers. There are well known parameters that will predict if your amp is a good match. Good Luck!
P.S. I've purchased many items without hearing but ALWAYS a good idea to hear first.

@lobinero - If you are making a major purchase from someone on the verge of retirement. Definitely ask if he is selling the business to anyone (who will support his legacy equipment). If he isn't, ask for schematics and parts list so you can provide to any technician who may work on your stuff in the future. Why would he say "no"?