Viola or violin? – The Violectric HPA-V550 [€2'399] doesn't care. As a balanced headphone amp with 2 x RCA and 1 x XLR inputs, it's instrument agnostic. To better adapt to various headfi loads, the gain for the 2 x 6.3mm and XLR4 outputs can be set via rear-panel dip switches. So can the gain on the RCA/XLR line outputs. Max power delivery into 50Ω is 6.4wpc. There's an Alps RK27 pot or, for a €600 surcharge, a 256-step switched-relay resistor array. A remote control includes with either version. Finish is black exclusively.