Is there any benefit for adding a preamp for my current system set

Short answer is yes, active preamp might give you a better sound. Interconnect cables between preamp and amp would also be very important, they should not be lesser than cables between source and preamp.

What exactly do you like about the MHA 100? You already aren’t using the DAC and now want to skip the pre as well. If you decide to use a separate pre then you’ll still be running through the MHA pre section if I understand the unit correctly. If instead you used the pre out on the MHA into a power amp you would avoid redundancy. I’m not saying a preamp won’t help. There are as many exceptions as rules, but in general, extra stages of gain and attenuation are likely to do more harm than good.

I’m not familiar enough with what you have now and what you’re looking for to gauge what type of improvement you’ll gain or even what your budget is. Give it a think and make a plan. Even if you can’t buy it all at once, at least you’ll know where you’re headed. 

thanks cat_...., I've recently started using the MHA 100's 50w output for speakers and have been really impressed paired with super efficient tower speakers.  My original buying motivation for the MHA amp was headphones.   Very pleased with that application.

But using the MHA only for headphone sessions seems an awful waste of a 50W class A amp.  And of course, those dancing blue gauges undoubtedly make the music sounds better.  LOL~!   Thus, my motive for attempting match additional gear to the amp.  And continue to multi-purpose the MHA.

I've come to conclusion that if going to make improvments on the front end, I'll have to support with the right gear and designate the MHA as headphone use only.  Cannot bi-pass the volume in the MHA.