If the tube has good venting like the SΩI Shorty, Broadside, Raven, etc. it's fine. You just need to do your homework and be very picky when selecting Tubes. With Tubes I build around 0.18Ω - 0.22Ω using Sony VTC5A's which are 25A, with parallels I build around 0.13Ω - 0.17Ω again with Sony...