Having a pair of speakers that are anorexic is really disappointing.  You spend a fair amount of time scaling up some designs you bought to go from 6" to 8" main drivers and adding a nice ribbon tweeter, thinking the result will be magical.
Then you end up with something that has no bottom end.  Depression sets in.
The parts I used are:
PRV Audio 8MR600X 8"
Dayton Audio PMPC-10 10uF 250V Precision Audio Capacitor
Dayton Audio PT2C-8 Planar Tweeter

I scaled up on the DNA plans I bought and presto. I get no bottom end.

I'm using my Zen amp and anything from mp3 to FLAC files.

Heck, I even tried my Pioneer Elite receiver.  

flat, thin, no bass does not even begin to describe how disappointed I am with the sound.

For those of you who have more and better experience with speaker design and building, do you have any suggestions to find the missing bass?

Thanks for the guidance. I will stuff the daylights out of the chamber until the new drivers arrive.  I ordered those drivers in hops that I'll finally get some bottom end.
If these work, my intention is to either wrap the cabinet with 3/4" walnut to get a good looking cabinet or to start from scratch and build new cabinets out of the 3/4 walnut.  Wrapping them is going to be the faster of the two options, but will mean they are going to weigh a ton.  But they will be so dead that they won't resonate.

The MDF gets me a cheaper material to work out the design and cuts so I can get something that sounds good before I get into making them look pretty.  I had really high hopes because I love the sound from the ribbon tweeters, but without any bottom end, these cabinets almost got scrapped.  I'm going to give them another try with the new drivers.

and I just ordered the new caps.

Again, thank you for the guidance!

Rich,

I'm glad that everything worked out for you. The Silver Flutes will get better and better and the tweeters crossing over higher will help the sound quite a bit.

I've slowly learned what all of those numbers that they show in the specs of speakers mean.

One big thing I noticed with the PVR's was that they didn't go very low and that they were 5 or 6 Db more efficient than the tweeter. That is noticable.

Anyway I'm glad that it sounds much better now.

Rich,
Why not try veneering the cabinets?

You can get really nice results and there are a bunch of different woods to pick from.

This is who I use.

https://www.oakwoodveneer.com/wood-veneer/

Yep, your already built MDF cabinets, a sheet of veneer and some glue is all you need to make 'em purty.

I'm never the voice of reason, kind of like the little devil that sits on your shoulder!

 Just read this thread, and the beginning just didn't make sense... until the driver being used was discussed. I had planned on trying out 3 different drivers, the Silver Flutes being the third driver to try out. I never made it past the second driver, which is the Mark Audio 12P. I first tried the Audio Nirvana 8" classic, and as the opposite of your experience there was way too much bass, and it was boomy. I don't even remember the midrange or treble. After about a day or two, that driver had to come out. The 12P is stunning even today after break in. I mean it sounded better than almost anything that I ever heard right out of the box. Now that the weather is agreeable, I will have the chance to finish up the cabinets and hear even better bass, since the cabinets will have an additional 3/4" layer of ply that has been damped with Acousti-coat from  Parts Express. The cabinet is made of  3/4" MDF, and the ply will be bonded over that.

I thank you, Rich for the information.
A comparison to the NFX sounds like a lot of fun!  I think I remember that they can go close to the back wall, so they save space.

Best wishes for whatever you decide to do.
Brian