Can "Dark"Sounding Speakers Be "Brightened"UP???

I second Wilsynet and Elizabeth in that the (main) problem you are hearing may not be the speakers. As Eugene81 reports, the speakers may be midranged focused but you may be able to improve things to an acceptable level by making other changes. My recent experience swapping around IC cables showed significant changes in the HF presentation between even very good cables. My overall experience has generally been that "darkness" (being a somewhat warmer than life mid-bass, less extended treble, and a focus on body and depth vs. resolution and detail) is sometimes a function of the electronics. Personally, to my preference, some of the best sounding gear I have owned had a touch of darkness.

Some of what other posters have said have got me thinking that you may just have a very warm midrange speaker that nothing can alter to the point where you will be satisfied.
OTOH I have a specific cable suugestion to try which are the all high purity solid silver cable called Silver Lace by Homegrown Audio. They are not overly expensive in the era of precious metal haording. If you have money to spare then the Silver version of the OHNO cast crystal silver cable may work as well or better.
I bought Jena labs pure copper for speaker speaker cables used they were still big money to me. I am pretty sure she is making a cast braided silver cable these days but again they aren't cheap. Jena is by no means very expensive compared to some others so it's not an outrageous compararitively either. There are other manufacturers that use it VanDenhalland several others (sp?) If you search you'll foind the others.
Ultimately all this tweaking I did, did not give me the sound I really wanted. I was fixed on keeping my JM Lab Focals and the I got a chance to buy some relatively potent tube monoblock poweramps.on the cheap I did so The search was over but I always wanted a 6SN7 preamp because I collect them and had only driver slots on an integrated to use them in,So a I snatched an AE-3 preamp(a Cary subdivision) which use 6SN7s to do the amplification and some other jobs in the pre.
I was simply delighted with the results all I wanted and more. My audio buddies always asked what did you do to those speakers they are sounding great ? I told them nothing.
So the posters sayit may be your amp might be right. Still others saying you can't make the seakers a bit leaner may be right. Thus do try several amps. My personal audio victory took years and lot of amps.
If you decide to try tubes get a modern pre or power amp they have sparkle and life and they aren't your Dads Fisher/ Pilot/ etc. I could hekp you optimize the tube choices if you take the plunge.
The amp should really have plenty of sizzle so it's surprising try a different pair speakers and see if it is the speaker. Otherwise the next question is what speaker should I buy.-Panels?? I don't like a lot of treble energy. I like speed transparency and stage wich my system has, it is by no means a sludgy dark overly warm sound. All the effort I (and $) I did spend was to rid my system of harshness and digital hash, even from my TT. Hey it's an adventure, If you buy the easy tweaks first go ahead but the DSP I looked up and it cost 900 euros, big money.
Good luck a lot of probably fun times await while you experiment your way into a more enjoyable system.

If a previous owner of the same speakers says that it just isn't that extended, then I'd go with that. The most important components in a stereo system are, in order: speakers, speaker amp synergy, amp, preamp, source.

There's only so much you can do to change the fundamental nature of the speakers, and speakers make the biggest difference.

just looked at this post if this has been answered...sorry.
If the crossover has an Lpad, with any pad on the tweeter, yes, you can reduce the amount of tweeter pad without changing the crossovers function. If the crossover has no pad or if the pad used was a single resistor figuring its resistance into the tweeter, then no, not easily, you would change the crossover design.

Back in the late 90's when I first heard the Concertinos at a Tweeter store, they were driven by a Marantz receiver (no idea on the model). Listened to a variety of material and I heard plenty of "sparkle" in the treble. Flash forward ten years and I finally purchase a used pair. I paired them with a VTL tube amp. Unfortunately, I heard much of what your friend describes. So basically I'm thinking that Concertinos are picky about amplification.