is a step up/down electric transformer bad ?

Should not cause a huge amount of difference as long as it's providing 100VAC; your home AC is going through a transformer outside.

I use a Japanese native (100V 50Hz) Sony TA4650 VFET amp all day on 115V 60Hz and never once had a problem (I replaced the diodes and check bias often).

Owned it since 1988, and it reigns as my best sounding SS amp; bar none.

The rule of thumb is that the transformer be rated for double the maximum that you are going to ask of it. If you go above that, the transformer will add distortion to the AC line and the results are both audible and measurable! and not good.

I use isolation or center-tapped (balanced), step-down transformers on almost all my gear, all rated for 115V, including sources and 2 class A monoblocks. The exception being a dedicated, class AB, bass amp. The current swings for the latter 4X400W amp would be tough, but not impossible, to keep up with but since everything else was isolated, it wasn't necessary. Even had a regulated (ferroresonant) iso tranny for the plasma TV for a while but it was too noisy.

The only sonic difference, as compared to wall outlet, should be less noise. If there is a change in the "character" of the sound, something might be wrong either way.

Kashimura TI-19 Transformer is okay.But it is made in China.Nissyo and Sanyo Transformers are made in Japan.Only works 115V to 100v.If you want to change your Luxman L590 to North American 115V.Send me a email.