Threshold, Parasound, Pass Labs or Nakamichi?

I have a question regarding my Arcam AVR350 and adding an amp.

I have been a novice hobbyist for over 20 years and have enjoyed the sound of a Nakamichi TA-2A, Adcom GCD-600 and 700, Nakamichi cassette deck and a number of KEF speakers. First and foremost, I am a diehard KEF fan. I have listened to various B&W 800 series, Wilson Audio and KEF Reference and have recently acquired a pair of KEF Reference 205/2s. I initially hooked them up to my little Nakamichi and loved the sound but obviously needed more power and needed to step up.

Unfortunately I also wanted the convenience of running my TV, DVD, and cable as well as my CD player and tape deck. I consider myself an amateur audiophile who loves music first and wants a home theater second. After a year of research, I found the Arcam AVRs were reviewed as the "best" receiver for sound. I acquired the AVR350 and was surprised. This may sound ridiculous but my 50 watt Nak seemed to produce a fuller and more 3 dimensional soundstage than the 120 watt, in stereo, Arcam. I do realize amps drive the speaker and am unsure of how many amps either of these units produce. I love the simplicity of the Arcam, the ease of use and what it does. However, the music sounds very accurate but "flat". The sound is much more 2 dimensional to me. The bass is significantly less pronounced with the Arcam also. I actually enjoyed the sound of my Nak better.

Hence, my question. I am considering purchasing a used amp and hoping to have the convenience of the Arcam pre-amp with the warmth, body, imaging and soundstage of a better amp. Obviously I am a fan of Nelson Pass's technologies and I am sure my dream system would consist of Pass Labs monos. Realistically, I am wondering if anyone would recommend any of the above amps based on their experience. I am considering a Nakamichi PA-7, Threshold s500 or s500e if I can find one, Parasound HCA3500 and I think my budget is still out of reach of a Pass Labs used amp.