Is this unit capable of storig backing tracks?

This question was answered with just a YES. Can anyone confirm this? It does not say it anywhere in the description that I could see.

Does it come with power adapter 9v 120 ac?

The BOSS GT-1000 comes with power adapter that you plug into your 120 volt outlet in the United States, if you purchased the unit in the United States. I cannot speak to any other form of power adapter such as something that might be sold outside of the U.S. All power parts are included so you just plug it in and start using it. I have enjoyed mine immensely, highly recommended.

Boss gt-1000. does anyone know what is the best output selection for evh amp? (there is no evh option in the list….) thanks

As with all of the Boss GT-xxxxxx pedalboards, they are a preamp. For years I would just run a cable from my GT-3 later my GT-10 (Left) output to the main input on my Fender Deluxe amp. Then one day I was at Guitar Center, the Boss rep was there. He instructed for the best control of the GT unit, plug directly into the "Power Amp In" input, bypassing all of the tone, volume and other controls. Once I did that, the GT became all that I needed. Internally on the GT, you may want to use something similar to the amp that you are currently using. This will alter the output into the amp. Though I have a Fender Deluxe, I used the JC-120 output setting. I have used the "Combo Amp" setting, but for some reason the tone just wasn't right. Bottom line is you'll have to play with it to get the sound you want. I wouldn't use the Line/Phones output setting though. I think you'll find with that the output setting might be too low for the amp GT's are great pedalboards. My brother has the GT-1000 and he loves it.