Go here:
http://www.whiteoakaudio.com/estore.aspx
Order, assemble, and install a new PL14_20 Rev G1 Dual Mono Control Board, at a minimum! Opt for the Phoenix edge connectors, they make assembly so much easier. This is the original upgrade and prevents the dreaded "Flame Linear" syndrome that destroys speakers. There is a great story about how this Control Board came to be...
For added functionality and safety order the PL700 Capacitor Assembly. The original capacitors are 40-45 years old and typically are good for 15 years.
If you want nirvana (who doesn't), order the PL700 Backplane Kit and the Backplane Wire Kit, as well as (12) MJ211295G and (12) MJ21196G output transistors to convert your 700B from quasi-complementary to full complimentary architecture. (This is an advanced skills kit, you have to be VERY good with a small tip soldering pencil to build the backplanes. If you don't feel 100% confident after looking at the kit, send it to one of us and we will build them for you, for a small charge and shipping.
For cosmetics, order the PL700 LED Light Board Kit in your choice of colors (I recommend warm white to duplicate the original PL lamp color).
At that point you will have a full White Oak Phase Linear (WOPL) amp that will deliver about 1000 Watts of the cleanest power you have ever heard. I would hold the WOPL amp up against any power amplifier. And if you like it loud (many of us refer to this as Mach 10), you will never experience anything like it.