For solid state the Creek 5350se is incredible at $1500 in th world of tubes the Rogue audio tempest is wonderful at $2195.
I would definately upgrade your speakers before considering an amplifier upgrade the NAD combo you have is actually very good and you would have to spend quite a bit on speakers before you begin to realize it's potential.
Nice speakers in the $1500 range are the Meadowlark Kestrals, overall one of my favorite speakers, the new Audio Physic Yara, used Audio Physic Spark III's if you can find a pair. I would upgrade the speakers long before I thought about upgrading your amplification.