I am also a retired broadcast sound engineer. I retired jsut as digital mixers were taking hold, even though the desks I used (AMS/Neve) were digital they still worked using analogue type levels, everything was referenced to zero level. It made switching from analogue easiere for us oldies. I still can't get used to setting up compressors and limiters with reference to dBfs. 
 In your case the natural place to change the levels if the signal is clipped is before the A to D which would be in the casette machine but as you have stated Tascam does not provide an adjustment. I think there is little point in reducing the digital signal before recording it as this would just reduce the dynamic range, might not matter with cassettes as the source. 
						
					
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