The Imagine T speakers are not bad, and your NAD 375 is a very good amplifier.

On the other hand, the Imagine T is not the last word in resolution.

I chose the Image T6 speakers over them because I found them more coherent from top to bottom. I had them for 2 years.

I replaced them with a pair of Gallo Acoustics Classico CL-3 speakers, and they will be a HUGE improvement over the Imagine T speakers!

They have clarity and resolution in spades; they have a unique patented tweeter that is wonderful, and high-excursion carbon-fiber drivers that are very responsive. 

But there is no need to take my word for it:

You can get a pair of them for a FREE 60-day home trial! (shipping paid both ways if you return them...which you won't...lol)

You will see for yourself; no risk. They are $1699 per pair, and that is a bargain.

They were originally $2395/pair before they decided to sell only factory-direct and eliminate dealer markup.

Read the article on them from The Absolute Sound last July; you can find it online.

The Imagine T2 is nearly comparable in sound quality, but they cost $3500/pair.

The NAD C375BEE is not the weak link in your system, but its DAC module might be. It is basically a $100-200 add on. You could get considerable additional capability and better sound in an outboard DAC for $1,000 or less. How noticible the difference would be is subject to various factors concerning total system resolution I think.

On the other hand, I can garantee that upgrading to better speakers will make a noticible improvement when connected to the NAD C375BEE. The C275BEE I'm using is essentially the power amp section of the C375, and I recently did a speaker upgrade such as you may be considering. The NAD does not seem to limit the new speakers very much, if at all.

The way I look at your situation is to either upgrade to speakers somewhere in the $2,000-$4,000 range, or replace the pre-amp and DAC portion of the C375 by connecting either a Grace m903, or a Benchmark DAC2, or a Mytek Stereo192 DSD to the NAD's power-amp main-in, each of these units available for under $2,000.

You could do both, but I usually like to  concentrate on one change at a time.

Don't get down about the PSB Imagine T's, nothing wrong with them at that level. It's just that if you want to invest in the next level of speaker capability, you have an opportunity with the NAD to go there. Or, upgrading your DAC could (depending on what you get) give you a slightly smoother sound with better tone, and with the same, or better detail from your Imagine T's.

I can't be of much help with selecting cables to enhance the music because I consider spending money on anything other than cables with good quality copper conductors and reliable terminations (not $100 WBT's) to be a waste. I'd rather put the money I have towards music, or components that actually do something audible. So, many of the cables I use are made up with Belden wire products in different configurations from Blue Jeans Cable. But yes, to answer your question,  I am pretty sure Kimber Hero cable would not hold a DAC back. Well maybe it might hold the $12,000 Light Harmonic DaVinci DAC back, I really don't know for sure on that one. They may have been using fancy cables when I heard it.

I will admit to occasional minor splurging on cables. I am using a .5m USB cable from Wireworld ($45), the Ultraviolet USB, simply because I like the fact that it's a flat cable with the power conductors separated farther physically from the signal conductors than most common USB cables. No other reason than that. I doubt it has an effect on the system sound, and if it does, I can not tell. I also bought my sister an Audioquest Evergreen 3.5mm-to-RCA cable ($33) as a gift for her new system. I could have gotten basically the same cables, though less colorful,  from Monoprice for probably less than $5, or $28 for the USB, and a stereo-mini made to length from Blue Jeans Cable.

I am not against anyone buying or selling high priced cables. I've seen some really nice looking stuff, and if I had the money, and was going for the bling, I'd be all over it. I think I had an insight a couple months ago at an in-store HiFi event. A high end cable manufacturer's representative did a quick listening demo first using their $100 USB cable, then playing the same unfamiliar recording using their $600 cable, after which he appeared to get consensus from the audience on the obvious improvement without actually questioning anyone. At one point during the second playing, I heard some particularly clear and wonderful sounding guitar notes. My first thought was that the $600 cable was worth every penny. But my second thought was "Hey, let me hear that first one again" I don't know how many people in the audience were convinced, but I wasn't.

Let me state, for anyone who truly hears audible effects of different thicknesses of silver plating, complex wire geometry, or voltage biasing of insulating materials, I commend you, and wish in a way I had your hearing ability.