What you are hearing is a lack of understanding of the mix process.
The Tascam indeed does not have mic pres but that would not contribute to the "dull" sound you are hearing.
What you need to understand is how to make a mix full and spatial.
That, takes time and practice to teach your ears the proper frequencies for the overall mix and instruments within.
Also mix techniques to get the overall level and clarity as well. Things like using multi banc compressors, reverbs, group channels and inserts on the master fader.
Remember that what sounds good to you in your environment may not sound good on someone else's speakers. Always listen to the mix in as many places as possibly before saying to yourself that it's the perfect mix(which is very hard to do on the first go about or the 5th go about!)