Thanks to all for the information.
I also posted this question on the R390a email list and got a reply from WA0HQQ who referred me to a report he posted on this site. It is very detailed with nice graphs comparing stock R390a filters with Longwave products. Link is below.
http://www.r-390a.net/Collins-Curry-Study.pdfFrom KH6U:
"I put a Dave Curry 6kc filter in one of my R-390As in place of
the 16 kc filter. It works very well and I have been thinking of changing
the 2 kc filter to one of his 2.5 kc filters. One of my 390As was free and
the other (a Rich Mich rebuild) was only $200 so I feel I can invest in
filters which are useful to me. The 16 kc was often too wide and the 2 kc
filter is always too narrow for SSB at least to my ear."
73,
Steve