Images aren't working for me. Maybe try a different way of uploading images. One did pop up briefly, but then disappeared before the page was finished loading. I did catch the manufacturers name and the model number and did some searching. The model number didn't get any results. The manufacturer was easy to find though and there are a lot of vintage amps under the brand names 'Prince' and 'Piggy' made by this company. Here's a written profile of the company from when it was making guitar amplifiers. ("Prince Tsushinkogyo" seventh one down, last on the list).
EDIT: I got the images up by reloading the page repeatedly. OOops! One just disappeared again!?! This manufacturer made mostly solid state amps and since this is a "mixing amplifier" (old style PA head) I'll suppose that it's solid state. Solid state from Japan in that era isn't going to have any collectors value. But the companies products are said to be pretty good and reliable in the article. I'd say if it works well then use it if you have a use for it. That's the limit of it's value. If it needs repair you're almost on your own. Though we can certainly help with any well structured electronics questions you're not likely to find a schematic or anyone with experience working on that particular model.
EDIT 2: I've been trying to correct the poor image loading and now I can't reload images onto your post.?. I get a bulletin that says "You've exceeded your allowance by blah blah blah". Maybe a daily upload limit? I'm uploading versions of your images on MY post here because I can't put them back onto your post other than the "add image" option. Which can't be viewed by every member here depending on their viewing circumstances. Sorry for the confusion.