Glad you found the issue , and yes , when you can't hear what is wrong or can't tell the difference but something is not right , the voltmeter and a scope works wonders.
Well assuming you have a decent amount of skill using the testing tools, with the scope you can measure voltages and freq, but the voltmeter is really simple.
With the scope you can see if your signal is clipped , have any spurious signal , have DC offset or is distorted in any manner , with a dual channel scope you can check phase alignment also.
There so many stuff you can do but those are the most common ones.
So how it sounds now.
Well assuming you have a decent amount of skill using the testing tools, with the scope you can measure voltages and freq, but the voltmeter is really simple.
With the scope you can see if your signal is clipped , have any spurious signal , have DC offset or is distorted in any manner , with a dual channel scope you can check phase alignment also.
There so many stuff you can do but those are the most common ones.
So how it sounds now.