Should I upgrade my preamp?

@ghdprentice ,1st, I would like to say I read the majority of your posts and enjoy your sentiment and opinions. However, you and I are very different listeners...and this is ok. I spent most of my life helping others build / upgrade their systems. You say you do not believe you must understand what your system is missing. Maybe the word understand is not the correct word. My experience shows the listener simply wants more out of their system, before the upgrade comes. This is the reason for the upgrade in the 1st place. To take a system, and simply exchange a new component for the sake of it, is not how I see it. A listener needs to know what his / her priorities are, with music and sound, in building a system, and I tried to help / teach this to the client. The 1st and foremost step in helping someone with their system build / upgrade, is to determine what sound / presentation do they want, to learn what their listening habits are, the music listened to, and of course, the environment they are in. I know you love AR tube gear, and SF speakers, and I understand why. This presentation might be fine for you, but not everyone. Admittedly, not my cup of tea, as I believe you might feel the same concerning my system. Why, because we both value certain aspects of recorded music reproduction. So, what is good for you, or me, is likely different than the next person. Most listeners I correspond with, are still so lost in achieving their holy grail. They have not figured out what they want...or know what is potentially available. A problem that exists ime, is that listeners do not know the difference between the reproduction of recorded music and live, unamplified music, and try to achieve that balance of both. I apologize for going on and on, but recommending anything to a listener who wants to upgrade, I need this information to help them further. Before I retired, I did have many happy clients, because I concentrated on what "their" likes, wants and desires were, and helped them get to that point by educating them to the best of my ability. Anyway, I hope this did not turn you sour in any way. Remember, ymmv, we are all different, and music listening should be fun. Listeners, mostly audiophiles, experience neurosis with this hobby, which is why I retired. Unfortunately, it is getting worse, imo, likely because of the world we live in. To the OP, good luck. Again, I apologize. My best, MrD.

@mrdecibel 

My experience with high end audio has been that each significant upgrade I have made of high end main stream audio components has resulted in a great improvement on my system. I agree that it is very helpful to know what sonic characteristics you value. But, you need an experience base to really understand what the choices are. In the case of a preamp and what we know about the poster, I think the ARC is a very safe bet for this person. Now if it was speakers or an amp, much more information would be required. 
 

I agree that we have different values… and that is fine. 

I think your speakers and phono cartridge are fine. The weakest link in my opinion is your cd player. The old ARC preamp has more potential and if you add a DAC and a good transport you can improve the performance of your system tremendously. 
In example Audiolab 6000CDT transport under $400 used and a Chord Qutest DAC around $1000. Or a PS Audio Directstream DAC going under $1800 now on TMR as a special. You can upgrade the preamp later. 
The LS 28 with your Denon cd player will be garbage in garbage out type of deal. 

I thought the 3.6 was the best per dollar speaker ever produced by Thiel. I liked them when driven by Krell and Levinson, which were available to me back then. Such an improvement over the 3.5. When Robert Harley reviewed the 3.6 speakers in '93, the LS2B was used throughout the review. He never stated it was a weakness in the system. You might want to consider having the LS2B serviced / updated. You still have not told us what sq you are looking to improve / change. The source is always an area to upgrade, as I agree with audphile1. I would also try different cables, if yours have those time alignment [email protected] good my comrade. Enjoy ! MrD.