Buy the cheapest filter you can find. Buy the most outrageously priced filter you can find. Take pictures through both and see if you can tell the pictures apart..
I see you're an advertiser - you don't by any chance sell cheap filters, do you???
(only kidding and I genuinely don't know what products or services you sell!)
Out of interest, I looked at what Ilford have to say on the subject:
"If your enlarger lacks a filter drawer, you will need to use mounted filters, by sliding them into an adaptor under the lens."
..... which I read as "best option,filter drawer"
Then, supporting Nicholas' opinion:
"The optical quality of the filters is high, so they may be used anywhere in the enlarger light path."
But also:
"If you use them below the lens, you must handle them with particular care to avoid finger marks and other blemishes."
So if they aren't new and/or perfect (which I'm sure all of ours are!) they're better off "up top".
http://www.ilfordphoto.com/aboutus/page.asp?n=117
I guess we must all pay our money (or not in the case of cheap filters) and take our choice!
Steve