Preamp upgrade

Opinion.....Pre amps should Always be Tubed....

Trying to assign a word to whether or how much better it may sound is a mistake.  Will it sound better? Most likely yes.  Will it sound much better in your system with your other present components and warrant a change? Again, probably yes.  Will it crush your current preamp?  Only you can decide.  I’ve listened to the current ARC Reference preamps several times and they’re very good.  I’m not sure how you would be anything but pleased.

To be honest unless you are Elon Musk price matters.

The exponential increase in price of let's say a Ref 6SE over your LS7 will not give an exponential increase of quality reproduction. 

Most of us in this hobby spend a LOT more money than one could justify chasing nuance and the thrill of a high quality new component. 

Therefore; a cost benefit analysis isn't the best way to examine value of Hifi gear. It's more an emotional purchase. In my opinion long after the price is forgotten the thrill of beautifully reproduced music and ultra high quality gear remains.  

Performance as concerns sound is foremost, but given the already curated list, I would assume any of the mentioned ones would be enjoyable. I would include in my consideration whether the maker was a large enough company to credibly provide support for as long as you would expect to own the component, meaning parts, a network of supported repair shops and if new, a reasonably long warranty. Unless you have money to burn, low-volume products from atelier-type producers are out. If you are buying used and at substantial discount from new purchase prices, you will have to weigh the risk of replacing the unit sooner than you would like for lack of parts and support. I have always thought AR made components that were well-engineered and well-constructed and durable, yet still, they are a company that has had its ups and downs in recent years (I still would buy their gear, FWIW.) Other outfits, it would depend.

@chenry I always look for point to point wired tube equipment that all the parts are easily sourced and available, there are many small manufacturers that do this. Some of the bigger manufacturers may use circuit boards that later are discontinued, so buying from a bigger well know manufacturer doesn't always mean you can easily get the item repaired.