Al is an eccellent rythm player just look at some DVD/video close ups. One in particular 'The Lost Concert', just watch his fingerings playing the less commonly used chords and in difficult keys for guitar without a capo,(only guitar players know what I mean), hardly any BB songs are simple 3 chord 12 bar.
As for his amp turned down it is noticed on the 'Beach Boys Concert' album when Mike introduces the band, he cannot be heard, (don't know why he didn't get applauded either,different than on the 'Lost concert' DVD).
This is a great example...all the guitar parts Al is playing during The Lost Concert are parts that David Marks put on the original studio recordings, or the same covers Dave had been playing with the band for the previous year and a half...Al is just playing Dave's old guitar parts as shown to him by Carl....except on Fun Fun Fun, which Al recorded himself....although David rehearsed that one with the BB's too while it was first being developed by Brian...he just left before they recorded it. This fact dawns on very few people. Al is a competent musician no doubt...but remember that the set the BB's play during the Lost Concert includes 100% guitar parts that Carl and David developed...not Carl and Al. And on the other hand, both David and Al will point out that although the BB's guitar sound was basically in place upon Al's re-arrival...the BB's vocal blend improved greatly with Al there.