While dicrotic filters may not fade they do get dirty. When I baught my DeVere 405 I found my prints were softer than previously, suspecting the filters were at fault I phoned Odessy sales enquireing if new ones were available, they were BUT the advise I was given was, they should not be needed unless broken, try cleaning them with alcohol on a cotton bud but be careful as they are a bit delicate, if you brake them we have them in stock.
I cleaned the filters and have had no more problems.
Hope this of help to all.
Regards Paul.