I was one of several piano players at the KH and at the assemblies (a rare honor I'm told). There were orchestras at the conventions which were eliminated about the same time the records came out. There was even a small orchestra at the dedication of an assembly hall were Knorr gave the talk, he loved it. Pianos/organs were still used in 1984.
The reasons given for ending live music included:
1) It was harder for congregations to find qualified piano players.
2) It was necessary to be "unified."
3) Too much time was spent rehearsing for the conventions that could be spent preaching.
I just think they wanted to make money selling the albums/cassettes (pre-1990).
Many congregations sold their pianos and others tucked them away in the corner and let them go out of tune.
I know a jw who attended a circuit assembly out by Wallkill in the mid 80's where a Bethelite played the piano and butchered the songs....no unity there.
Jws wanting to marry in the KH found it hard to time the walk down the aisle to recorded music and brought in electronic pianos and rounded up piano players that still remained (of course with the elders "permission." It was easy to get that when it was an elder's daughter or son getting married.)
I know an active jw that told me the new songs were horrible and indicated that the musical ability of those at Bethel had hit a new low.