What can I improve in my audio system for better s

I have lossless FLAC music ripped from cd's and music I purchased from HD tracks, I am using Macintosh MX 121 processor, Krell S1500 7 channel power amp, 7.1 SYSTEM, B&W 802D speakers for front and center, Monitor Audio for side and rare channels, Crystal and Audio Quest for interconnect and Tara Lab for Speaker cables. I am using JRiver to play this music from my computer, which resides on Lenovo Q180 small foot print PC with build in HDMI which is connected to my processor via this HDMI. I don't know why but I feel that this PC is the least expensive equipment I have, and is the weakest link in my entire system. There is nothing wrong with the system. But, it always bugs me that I can improve this weak link. I like the fact that while playing music I can see the album cover and track information on the monitor. But still I am curious to know, if possible, how I can improve the sound quality. Since I have a McIntosh processor with built in DAC, I don't want to add another DAC (by the way I tried using Moon DAC, it was no-ware close to McIntosh MX121, ended up selling it on eBay). I am not even sure, while using HDMI port does better sound card matters? Or can I enhance anything in my system to improve the sound quality or am I looking for unrealistic Magic? When I purchased this system, my friends were so surprised from the money I spent, there first question to me was "Does singer comes out of these speakers to sing for you?" I guess that's what I am expecting; I believe I am missing those emotions that we get from live performance.
I would replace the Monitor Audio speakers with B&Ws that closely match your current ones. This will provide you with proper timbre matching and will greatly improve the overall quality of your soundfield. This is not a subtle improvement...

X2 very important??
Trens. Experiment with ac power isolation and conditioning- for everything! Probably put smiles on your face : )
Are your speakers capable of great dynamic swings? Can they play 3/4 times as loud as you would listen? Does your amp play that loud without using more than 30% of available dynamic power? That's needed to give you at least a natural relaxed sound to help with the emotion. Live music is incredibly dynamic.
There are so many things to do to improve a system. It can be as simple as swapping resistors in components and speakers. Shinko resistors have an emotional sound to them IMO. Caps - so many opinions but the copper foil ones all add a nice musical emotional sound. Then filter chokes in amps and preamps to filter AC noise can take many components to a different level. That provides a black background so you get more resolution, separation, and micro to macro dynamics.
I was reading Krell and B&W and thinking bright, bright. And your Crystal and Taralabs cables will just let through whatever is fed to them. To tame the 802D's & add some delicacy I would consider PAD cables which are warm, but on the dark side of neutral. Also Electrocompaniet and Rowland amps work well with the 802D's, though I could imagine Modwright also working well.
Today I got Bel Canto uLink, I connected and believe or not first thing came out of my mouth was Woooow. I have over $60K invested in my Audio Video equipment and this tiny $675 just amazed me. No more noise in the music, just music. So sharp, so open, so deep, just Wooooow.