Power tubes usually do require break-in. I've frequently found that the top smooths out and the bass developes after 24 to 100 hours of operation, BUT.....
Proper biasing is manditory and the bias setting for the 6L6's would be quite different than the 5881's. I'm not up to speed on the bias arrangement for the Onix but if its not auto bias (as I suspect) you should contact the distributor, referred to earlier, and find out how to bias in your new tubes.
Proper biasing is manditory and the bias setting for the 6L6's would be quite different than the 5881's. I'm not up to speed on the bias arrangement for the Onix but if its not auto bias (as I suspect) you should contact the distributor, referred to earlier, and find out how to bias in your new tubes.