The CAT amps sound great and will sound way better than your Krell in most set-ups, but they are extremely finnicky because of their hot-rod design. They require constant monitoring of bias settings and blow tubes easily, which requires new resistors to be soldered in. Don't buy one unless you know how to use a solder iron, have a lot of patience and have a lot of money for new output tubes.
The ARC amps use a lot of negative feedback and are hard to get to sound good as a result. They produce a great deal of heat as well (especially the 600 -- you need a separate room for them).
The above suggestion is good, but perhaps the 90 watt Lamm tube amp is better for WattPuppy 7's -- they drop to near 3 Ohms in the bass and need real current.
Another option is a VAC 70/70 Mk. III or Signature, as they can drive tough loads - they are easy to own because they auto-bias, have a tube shut down feature for tubes that fail, and use 300B's that provide really long life.
All of the above amps are very well made.
Stay away from Chinese tube amps -- the power supplies and output transformers are not robust enough for tough loads like WattPuppys.