Question on cable selection for Vandersteen 1

I have a tube amp coupled to a pair of Vandersteen Model 1 speakers coupled to 15/35 watt tube amplifier. They sound pretty good, but I am using really cheap speaker wire. I am wondering which is the better choice: 14 mm or 16 mm cables with banana plugs? Will the 16 mm cable give me too much base or is the higher diameter the better one in general? I play mostly rock.
Do you mean 14/16 mm or 14 *GAUGE*? Most wire size is given in gauge, and the lower the number, the bigger the cable. So 14 ga. would have less resistance (and possibly more bass depending on a lot of other factors) than 16 ga.

What are you willing to spend on cable? There are some speaker cables specifically made for tube amps from Transparent and Audio Magic, but they're pricy.

Otherwise, I'd just get some Audioquest GBC on closeout here. You'd be hard-pressed to get better for anywhere near that money.
Alpha-Core Goertz is reported to work well with Vandersteen. They're reasonably priced, and Alpha-Core offers a generous free trial period.
I'm using Anti-Cables on my Vandersteen 5A's with great success. Give them a try, you'll be surprised. They have a money back satisfaction guarantee.
My Vandersteen 1C's are connected to my Rogue Audio Cronus integrated amp (EL-34 tubes) with AudioQuest Type 6+ cables, and the synergy is wonderful. Give these cables a try. You may be very happy with the sound. Happy Listening.
Vandersteen used Audioquest cables for everything they do and they have for many many years.  I have the Castlerock biwired, but purchase what you can afford of any Audioquest cable (make sure they are authentic AQ cables if you purchase used as there really are a ton of fake ones on the market) and you will be very happy.  Cardas is another brand that works well with them.  
Audioquest, Signal Cable or Silent Source, are all excellent for Vandy speakers.