Revox PR99 reference level
When I worked at Teac the Technicians had a discussion early on about how the bias should be set and we discussed the two ways to do it. We found and we told later on that the service manual is written for Technicians and often times with the assembly line in mind. Overbias is a fast and sloppy way to do the job to turn out more units from the line.
It is not found to be the way for exacting calibration. That can only be by either spot frequencies or sweeps as is available on a device like the Audio Precision One plus analyzer. Here the 1KHz and 10KHz are compared and then once those are close to each other further modification allow the frequency response be contoured to go out to extended frequencies such as 31.5KHz at 15 IPS or in some deck 28KHz.
Of course you can not hear that but it shows what a machine can do. I have had factory decks come in that would be 3 dB down at 9KHz.
Is this what you call Hi Fi? I don't When I got done with the machine it went out to 22KHz at 0 Vu at 7.5 IPS. In all my years of working on decks I have had no negative comments about my calibration process either at Teac or Sony or after. In fact people often tell me they sound better than they were new. I suspect that is correct as they were probably set wrong from the factory which is common. The manufacture only provides you with a deck that works- it does not say it is optimum adjusted. That is why there is such a long list of tape in the manuals as it is set up somewhere in the range of all those tapes- none are perfect.
I have not used over bias since 1982. I can send sweep results to those who wish to see them by E mail bit in the past I have already posted some of those. I use plus or minus 2 dB not 3dB. Some machines come out to plus or minus 1dB if it were not for the head bump.