Crispness, is usually lost when the negative is not dead accurately grain focused. You can have a really soft grade from filtration, that is as sharp as a tack, you can also have a really hard grade from filtration, that is as sharp as a tack.
The amount of softness or hardness, or wished for contrast in your print, is usually completely different to a soft or out of focus print.
One trick of darkroom printers in commercial labs, was to knock the focus out a poofteenth, one would then (usually) have a soft focus print at whatever grade of contrast you chose. That said, making the focus slightly off, will (in my experience) give you an apparent softer or lower contrast of any given level of chosen contrast, than if the negative is grain sharp.
When the grain is focused with a glass held negative, in a correctly aligned enlarger, which includes the baseboard being in alignment with the negative, you have almost no possibility of obtaining soft, or non crisp prints. Unless you wish to deliberately have soft, or non crisp prints.
Within reason, having a glassless held negative, especially 135 format, should not make that much of a difference. Apart from the fact it may, with some curved negatives, cause apparent loss of crispness in areas that are not in absolute alignment.
With enlargements over 15 x the size of your negative, then it would be normal (if possible) to use a glass held negative. If not possible, then consider holding the negative on either side (where the rebates/sprocket holes are) with removeable magic tape. This tape does not damage film, does not, in my experience, leave any residue, yet allows you to work quickly and with more control in obtaining a more flattish negative. that tape is available in various widths, the width I use is the narrowest available at 12.7mm in width.
I use this tape myself, having first encountered it around 30+ years ago in a commercial lab where I was printing. I personally use it for all 135, 120 formats in my own enlarger using glassless negative holders and also when doing 4x5" enlarging, where I wish to align the negative for whatever reason on the glass holder.
More or less filtration, or no filtration, should, all things being equal, make no difference to the crispness of your print, just the chosen conrast grade should be the difference.
There are many variables required to be in alignment with each other in order to have crisp prints, by eliminating each possible variable, you should be able to narrow down your apparent problem.
Mick.