Well, here is the problem with changing the EQ on the record side of the B77/PR99 or pretty much any other serious R2R. Doing so only makes sense if you have the possibility to do a full adjustment of your R2R.
The common method is first to adjust your playback frequency characteristic using a reference tape. For the B77/PR99, that is fixed and pretty accurate, so that makes a change from NAB to IEC fairly simple as described in the article.
On the recording side it is not that straight forward, because part of the recording EQ is adjustable. To adjust an R2R on the recording side, you first set the bias level (which cannot be done properly without some test and measurement equipment) and then you will adjust the EQ for flat response over the tape. As a final optimization you then tweak the bias level and the EQ simultaneously.
So changing the record electronics to match the IEC EQ circuit is easy, but it will be completely out of adjustment at that moment. So unless you have the possibility to adjust the R2R (or have it adjusted) afterwards, changing your record EQ will put you further away from your goal of being able to make decent recordings.
In that case it would be better to leave the EQs as they are on both the record side and the playback side and make only those changes on the playback side that improve the sound quality. You can then play IEC tapes with the new passive circuit I just posted on the LAOCAS website and you can still playback your own recordings with the NAB EQ from the machine itself.
PS. Keep in mind that IEC recordings at 15ips are only slightly better than NAB recordings at that speed. The difference only pays off with either live recordings or with extremely good source material. If you would record at 7.5ips, NAB is actually a better EQ for modern tape formulas than IEC is.
