I like the 6 spring tank, because more springs means more delays and more complexity to the reverb sound. However, I prefer the sound of a medium delay tank rather than the standard long delay. The reason is that it sounds more natural and less sproingy... Better reverb quality can be had from good digital boxes for recording or from a real plate reverb, but neither are practical for use with a guitar amp (a real plate reverb is enormous
Accutronics makes a 6 spring medium delay tank, model # 9AB2C1B. Their website
gives the meaning for each position in the part #, allowing you to specify something other than typical.