BRYSTONs are just fine : they are built to a very high standard and they simply pass through what is fed to them, with a minimal amount of further bespoke sonic signature colouration.
But like every other contender and pretender amp out there, they may not be a suitable match with the rest your system, as you search for that ethereal system synergy. ( ,,,, and that is all bits including your upstream source, your cables, and your downstream speakers …. Each with its own bespoke sonic signature forming part of your unique system sonic alchemy…) All the personal prior post characterizations with wide experience variances metaphors of “thin” vs “(insert your own) “ simply reflects and validates this.
if you are still married to your current speakers, then choose wisely, starting with doing your research, and then experiment with personal auditions to sort out the contenders from the pretenders.
afterwards, you may eventually reevaluate what’s what in total , and actually consider moving on from your current speakers …. This is a journey and no quick fix one-size-fits- all exercise.