I have no experience with the tandberg, (and I have learned that TWL has good and honest oppions that are trustworthy) but I have a lot of experince with a lot of cassette decks of many brands, and mostly naks.
While I do believe it is true the naks are the best, there is a compatibility issue. The best cassette recordings I have ever made have been on 3-head naks, and I consider a good recording one that works on all decks for playback. With a cassette you are dealing with both the quality on the tape and the ability of the deck to reproduce it. With a cassette, tracking is the biggest issue, and if the adjustments are not the same deck to deck, things fall apart quickly. 3-head naks have a VERY narrow tolerance that allows them to be capable of greater fidelity, but also makes recordings made on them to be more demanding for playing on other decks.
The most critical is the azimuth. That is the alignment of the head gap to the tape. It should be at a perfect 90 degree, but more importantly should be played back the same as it was recorded. If you play it back at a different adjustment, things get out of phase and you also lose highs frequecy. Without getting to into it, playing back with a narrower head gap than what was recorded is more forgiving than playing back on a wider gap than what was recorded. The narrower the head gap, the more information you can put on tape, but the harder it is to track. If you have a tape that was recorded on a narrow gap that has more information, it will sound better, but be more suseptable to misalignment.
The dragon is the flagship of the naks, in part because it continully adjust the playback head to be in alignment. It is also a very hard deck to keep in proper alignment because of its complexity, and finding people skilled at this is becoming impossible. It is also time consuming, and expensive. When buying a nak you should be prepared to have to possibly spend money on having it properly tuned. If it is, It will be the best deck you can get- if it isn't, you may be better served with a lesser capable deck.
Both the rx-505 and the dragon use heads with a very narrow gap and of the same tolerance. The dragon also has tone generaters so you can monitor the adjustment and make minor adjustments as you want, but if the dragon gets out of adjustment or gets in need of a tune-up, you are out of luck unless you are willing to spend 300 to 1000 to get it working right. The rx-505, while capable of nearly the same performance, has no tone generaters and so makes it harder to see how it is working, and thus harder to use, but cost less to maintain. The dragon is the best you can get for playback, but there are a lot of other naks that will do as good if you can keep them tuned, and better choices than the rx-505.
If you can be be more specific on your requirements, what you want to do and how much you want to spend, and what you have available, perhaps I (we) can help you better.
