WILL IT MAKE A DIFFERENCE?

@immatthewj I use Blue Jeans Cables and home brew speaker cables.  My speakers are Shahinian Diapasons and two Double Eagle Subwoofers, giving me two subs per channel.   I mounted them on two steel stands filled with lead shot.  As I said, the tube sound is a little compressed and a little bright with new tubes in the CJ Premier Fives @ 200W/ch, but after 75 to 100 hours or so, they mellow right out and sound excellent.  They do last a long time, at least the way I use them.

I have lived with the Diapasons & Double Eagle combination now for over 40 years, although they have had factory upgrades along the way.  They are the only speakers I found I can live with day after day and never tire of hearing them.   I can't say that of other speakers I have owned or heard.

While we are on the subject of equipment - my main preamp is the CJ, I also have a revamped Spatial TVA-2, the last model they made but never made it into production, a Parasound preamp, and a preamp of my own design.  I was on Spatial's Technical Staff, so everything they designed had my hands in it, which is why I have one.  I have two other amplifiers, a four channel Marantz Class D (450 W/ch into the subwoofer impedance) for use on the subs, and another Parasound 2275 #2 that I will soon replace the Marantz unit with.   I only have one of those, so picking up another one is next on the list. 

Hello jiii 12.  You have good gear. I think you may have a problem that can be solved by changing/upgrading cables. Here's a cheap and easy experiment: Get some ordinary 14 gauge zip cord (lamp cord) from a local hardware store. Get enough for two speaker cables. You are going to replace you speaker cables with the zip cord. Don't worry about banana plugs or spades. Just strip 1/2 - 3/4 of an inch of wire from both ends, twist the strands together, and stick 'em in the holes in the amp/speaker terminals. Tighten down on them. If you hear a difference, cables will solve your problem. All you are listening for is a difference. The lamp cord should sound worse. If it sounds better, you have solved the problem. If it sounds worse, you know your system is sensitive to cables (it should be). Now you enter the exciting search to find the RIGHT cable. See if dealers will loan you test cables to try out. You can purchase used cables on line. Don't spend more than you need to. Used cables are probably 'broken in,' but maybe not. Don't make any final decisions until the cables have been played at least 50 hours or so. Start with speaker cables, then move on to interconnects if you still seek improved sound. USB cables are very important. Is that how you streaming music reaches you? Does the problem occur only when streaming?  How do LPs sound? If LPs soound better, change the USB cable, if present. Got the idea?  Happy Listening!

Personally I'd get a sub or two before you do anything. Tubes or not, it'll reinforce the lower octaves and help create a better soundstage. SVS is very inexpensive for what what you get. 

As @boomerbillone said above, I would look at cables, both power and interconnects. Beg, borrow and demo a whole load. You have the PSM156 (great choice by the way), try their cables, the ultimate is very good.

And my standing recommendation to apply fo.q thick TA-102 tape to all your capacitors in your system. Cant emphasize enough how good that stuff is

Finally double check your speaker positioning, that can also help and is free

@op - try another speaker from a BBC influenced manufacturer like Harbeth or Spendor. That will tell you very quickly if the speaker is an issue for you. I haven't heard the Fritz speaker but I did have a pair of ProAc's which used a very similar driver combination and which were superb but did have a bit of raggedness around in the upper midrange. I appreciate that the Fritz speaker uses a different and unconventional crossover - but that makes it even more of an unknown quantity.  BTW, I dislike generalisations about tube and solid state SQ as it can be very misleading in practice when it comes to individual designs.